Rename mp_parser_hint to mp_size_hint, add mp_check_uint, mp_check_map.

mp_parser_hint contains a mapping between hexadecimal msgpack type
code and type length. mp_size_hint sounds like a better name for it.

Introduce mp_check_uint(), mp_check_map() to check the stream
specifically for MP_UINT and MP_MAP (mp_check() only tells you
if the stream has a valid mspgack content, and mp_typeof() doesn't
check the stream size).

The other difference between mp_check_map() and mp_check()
is that mp_check_map() only checks the map header.