Fix -Wclass-memaccess warnings/errors

Recent GCC versions warn about using memcpy/memmove to
write to a class pointer (-Wclass-memaccess).

Avoid the warnings by casting to void* first.

Closes #1086.
Closes #1205.
Closes #1246.

include/rapidjson/document.h

@@ -1513,7 +1513,7 @@ public:
         MemberIterator pos = MemberBegin() + (first - MemberBegin());
         for (MemberIterator itr = pos; itr != last; ++itr)
             itr->~Member();
-        std::memmove(&*pos, &*last, static_cast<size_t>(MemberEnd() - last) * sizeof(Member));
+        std::memmove(static_cast<void*>(&*pos), &*last, static_cast<size_t>(MemberEnd() - last) * sizeof(Member));
         data_.o.size -= static_cast<SizeType>(last - first);
         return pos;
     }
@@ -1716,8 +1716,8 @@ public:
         RAPIDJSON_ASSERT(last <= End());
         ValueIterator pos = Begin() + (first - Begin());
         for (ValueIterator itr = pos; itr != last; ++itr)
-            itr->~GenericValue();       
-        std::memmove(pos, last, static_cast<size_t>(End() - last) * sizeof(GenericValue));
+            itr->~GenericValue();
+        std::memmove(static_cast<void*>(pos), last, static_cast<size_t>(End() - last) * sizeof(GenericValue));
         data_.a.size -= static_cast<SizeType>(last - first);
         return pos;
     }
@@ -2032,12 +2032,7 @@ private:
         if (count) {
             GenericValue* e = static_cast<GenericValue*>(allocator.Malloc(count * sizeof(GenericValue)));
             SetElementsPointer(e);
-RAPIDJSON_DIAG_PUSH
-#if defined(__GNUC__) && __GNUC__ >= 8
-RAPIDJSON_DIAG_OFF(class-memaccess) // ignore complains from gcc that no trivial copy constructor exists.
-#endif
-            std::memcpy(e, values, count * sizeof(GenericValue));
-RAPIDJSON_DIAG_POP
+            std::memcpy(static_cast<void*>(e), values, count * sizeof(GenericValue));
         }
         else
             SetElementsPointer(0);
@@ -2050,12 +2045,7 @@ RAPIDJSON_DIAG_POP
         if (count) {
             Member* m = static_cast<Member*>(allocator.Malloc(count * sizeof(Member)));
             SetMembersPointer(m);
-RAPIDJSON_DIAG_PUSH
-#if defined(__GNUC__) && __GNUC__ >= 8
-RAPIDJSON_DIAG_OFF(class-memaccess) // ignore complains from gcc that no trivial copy constructor exists.
-#endif
-            std::memcpy(m, members, count * sizeof(Member));
-RAPIDJSON_DIAG_POP
+            std::memcpy(static_cast<void*>(m), members, count * sizeof(Member));
         }
         else
             SetMembersPointer(0);