jsonapi type fixups

I have this patch series that fixes up the types of the new incremental
JSON API a bit. Specifically, it uses "const" throughout so that the
top-level entry points such as pg_parse_json_incremental() can declare
their arguments as const char * instead of just char *. This just
works, it doesn't require any new casting tricks. In fact, it removes a
few unconstify() calls.

Also, a few arguments and variables that relate to object sizes should
be size_t rather than int. At this point, this mainly makes the API
better self-documenting. I don't think it actually works to parse
larger than 2 GB chunks (not tested).