Add dedicated internal types for Int64 and UnsignedInt64. This makes it
a bit more straightforward to work with 64-bit numbers (instead of just
implicitly storing them as doubles.)
This was a workaround to be able to build on case-insensitive file
systems where it might get confused about <string.h> vs <String.h>.
Let's just not support building that way, so String.h can have an
objectively nicer name. :^)
This way, primitive JsonValue serialization is still handled by
JsonValue::serialize(), but JsonArray and JsonObject serialization
always goes through serializer classes. This is no less efficient
if you have the whole JSON in memory already.
This patch adds JsonValue, JsonObject and JsonArray. You can use them to
build up a JsonObject and then serialize it to a string via to_string().
This patch only implements encoding, no decoding yet.
