GenericValue: improve copying performance

The GenericValue "copy" constructor (with Allocator) uses a temporary
GenericDocument object to perform the deep copying with the provided
allocator.  This leads to the temporary allocation of the `Stack`
memory, even in case of shallow values (numbers, etc.).

This patch improves the performance of this operation by only resorting
the the SAX Handler implementation in case of Array or Object values.

include/rapidjson/document.h

@@ -1896,9 +1896,26 @@ template <typename SourceAllocator>
 inline
 GenericValue<Encoding,Allocator>::GenericValue(const GenericValue<Encoding,SourceAllocator>& rhs, Allocator& allocator)
 {
-    GenericDocument<Encoding,Allocator> d(&allocator);
-    rhs.Accept(d);
-    RawAssign(*d.stack_.template Pop<GenericValue>(1));
+    switch (rhs.GetType()) {
+    case kObjectType:
+    case kArrayType: { // perform deep copy via SAX Handler
+            GenericDocument<Encoding,Allocator> d(&allocator);
+            rhs.Accept(d);
+            RawAssign(*d.stack_.template Pop<GenericValue>(1));
+        }
+        break;
+    case kStringType:
+        if (rhs.flags_ == kConstStringFlag) {
+            flags_ = rhs.flags_;
+            data_  = *reinterpret_cast<const Data*>(&rhs.data_);
+        } else {
+            SetStringRaw(StringRef(rhs.GetString(), rhs.GetStringLength()), allocator);
+        }
+        break;
+    default: // kNumberType, kTrueType, kFalseType, kNullType
+        flags_ = rhs.flags_;
+        data_  = *reinterpret_cast<const Data*>(&rhs.data_);
+    }
 }
 
 RAPIDJSON_NAMESPACE_END