Adjust ext type code handling to match spec

The msgpack specification states that the range for the `ext` type is [-128,127], with [0,127] being the user-defined range.  This PR does the following:

- Updates `msgpack_packer:pack_ext/3` to accept type codes in the range of [-128,127].
- Adds a test case ensuring that all such codes are accepted (and that codes outside the range are not).
- Updates encoding/decoding of `ext` type codes to be explicit about signedness.
- Changes the type code for `ERLANG_TERM` (in `msgpack_term`) to be inside the allowed range.