short string optimization

Since the payload (the `Data` union) of the current implementation of `GenericValue` is `12 bytes` (32 bit) or `16 bytes` (64 bit) it could store `UTF8`-encoded strings up to `10` or `14` chars plus the `terminating zero` character plus the string length:
``` C++
    struct ShortString {
        enum { MaxSize = sizeof(GenericValue::String) / sizeof(Ch) - sizeof(unsigned char) };
        Ch str[MaxSize];
        unsigned char length;
    };  // at most as many bytes as "String" above => 12 bytes in 32-bit mode, 16 bytes in 64-bit mode

```

This is achieved by introducing additional `kInlineStrFlag` and `kShortStringFlag` flags. When setting a new string value in `SetStringRaw(s, alloc)` it is first checked if the string is short enough to fit into the `inline string buffer` and if so the given source string will be copied into the new `ShortString` target instead of allocating additional memory for it.

include/rapidjson/document.h

@@ -1247,12 +1247,12 @@ int z = a[0u].GetInt();             // This works too.
     //!@name String
     //@{
 
-    const Ch* GetString() const { RAPIDJSON_ASSERT(IsString()); return data_.s.str; }
+    const Ch* GetString() const { RAPIDJSON_ASSERT(IsString()); return ((flags_ & kInlineStrFlag) ? data_.ss.str : data_.s.str); }
 
     //! Get the length of string.
     /*! Since rapidjson permits "\\u0000" in the json string, strlen(v.GetString()) may not equal to v.GetStringLength().
     */
-    SizeType GetStringLength() const { RAPIDJSON_ASSERT(IsString()); return data_.s.length; }
+    SizeType GetStringLength() const { RAPIDJSON_ASSERT(IsString()); return ((flags_ & kInlineStrFlag) ? data_.ss.length : data_.s.length); }
 
     //! Set this value as a string without copying source string.
     /*! This version has better performance with supplied length, and also support string containing null character.
@@ -1320,7 +1320,7 @@ int z = a[0u].GetInt();             // This works too.
             if (!handler.StartObject())
                 return false;
             for (ConstMemberIterator m = MemberBegin(); m != MemberEnd(); ++m) {
-                if (!handler.String(m->name.data_.s.str, m->name.data_.s.length, (m->name.flags_ & kCopyFlag) != 0))
+                if (!handler.String(m->name.GetString(), m->name.GetStringLength(), (m->name.flags_ & kCopyFlag) != 0))
                     return false;
                 if (!m->value.Accept(handler))
                     return false;
@@ -1336,7 +1336,7 @@ int z = a[0u].GetInt();             // This works too.
             return handler.EndArray(data_.a.size);
     
         case kStringType:
-            return handler.String(data_.s.str, data_.s.length, (flags_ & kCopyFlag) != 0);
+            return handler.String(data_.GetString(), data_.GetStringLength(), (flags_ & kCopyFlag) != 0);
     
         case kNumberType:
             if (IsInt())            return handler.Int(data_.n.i.i);
@@ -1365,6 +1365,7 @@ private:
         kDoubleFlag = 0x4000,
         kStringFlag = 0x100000,
         kCopyFlag = 0x200000,
+        kInlineStrFlag = 0x400000,
 
         // Initial flags of different types.
         kNullFlag = kNullType,
@@ -1378,6 +1379,7 @@ private:
         kNumberAnyFlag = kNumberType | kNumberFlag | kIntFlag | kInt64Flag | kUintFlag | kUint64Flag | kDoubleFlag,
         kConstStringFlag = kStringType | kStringFlag,
         kCopyStringFlag = kStringType | kStringFlag | kCopyFlag,
+        kShortStringFlag = kStringType | kStringFlag | kCopyFlag | kInlineStrFlag,
         kObjectFlag = kObjectType,
         kArrayFlag = kArrayType,
 
@@ -1393,6 +1395,12 @@ private:
         unsigned hashcode;  //!< reserved
     };  // 12 bytes in 32-bit mode, 16 bytes in 64-bit mode
 
+    struct ShortString {
+        enum { MaxSize = sizeof(String) / sizeof(Ch) - sizeof(unsigned char) };
+        Ch str[MaxSize];
+        unsigned char length;
+    };  // at most as many bytes as "String" above => 12 bytes in 32-bit mode, 16 bytes in 64-bit mode
+
     // By using proper binary layout, retrieval of different integer types do not need conversions.
     union Number {
 #if RAPIDJSON_ENDIAN == RAPIDJSON_LITTLEENDIAN
@@ -1433,6 +1441,7 @@ private:
 
     union Data {
         String s;
+        ShortString ss;
         Number n;
         Object o;
         Array a;
@@ -1463,11 +1472,19 @@ private:
 
     //! Initialize this value as copy string with initial data, without calling destructor.
     void SetStringRaw(StringRefType s, Allocator& allocator) {
-        flags_ = kCopyStringFlag;
-        data_.s.str = (Ch *)allocator.Malloc((s.length + 1) * sizeof(Ch));
-        data_.s.length = s.length;
-        memcpy(const_cast<Ch*>(data_.s.str), s, s.length * sizeof(Ch));
-        const_cast<Ch*>(data_.s.str)[s.length] = '\0';
+        Ch* str = NULL;
+        if(s.length < ShortString::MaxSize) {
+            flags_ = kShortStringFlag;
+            data_.ss.length = s.length;
+            str = data_.ss.str;
+        } else {
+            flags_ = kCopyStringFlag;
+            data_.s.length = s.length;
+            str = (Ch *)allocator.Malloc((s.length + 1) * sizeof(Ch));
+            data_.s.str = str;
+        }
+        memcpy(str, s, s.length * sizeof(Ch));
+        str[s.length] = '\0';
     }
 
     //! Assignment without calling destructor
@@ -1480,9 +1497,16 @@ private:
     bool StringEqual(const GenericValue& rhs) const {
         RAPIDJSON_ASSERT(IsString());
         RAPIDJSON_ASSERT(rhs.IsString());
-        return data_.s.length == rhs.data_.s.length &&
-            (data_.s.str == rhs.data_.s.str // fast path for constant string
-            || memcmp(data_.s.str, rhs.data_.s.str, sizeof(Ch) * data_.s.length) == 0);
+
+        const SizeType len1 = (flags_     == kShortStringFlag) ? data_.ss.length     : data_.s.length;
+        const SizeType len2 = (rhs.flags_ == kShortStringFlag) ? rhs.data_.ss.length : rhs.data_.s.length;
+        if(len1 != len2) { return false; }
+
+        const Ch* const str1 = (flags_     == kShortStringFlag) ? data_.ss.str     : data_.s.str;
+        const Ch* const str2 = (rhs.flags_ == kShortStringFlag) ? rhs.data_.ss.str : rhs.data_.s.str;
+        if(str1 == str2) { return true; } // fast path for constant string
+
+        return (memcmp(str1, str2, sizeof(Ch) * len1) == 0);
     }
 
     Data data_;