Finished v5 spec; passes MessagePack test suite

Finish implementing v5 types (bin, ext, etc.)

Added typedefs for struct cmp_object_s and struct cmp_ctx_s; they save
space and users will want to do it themselves anyway

Updated README.md

Moved MP_* types to cmp.h; they're in cmp_object_t anyway, so now they
can be more than just opaque numbers to users; also refactored them to
CMP_* since they're visible now, I may refactor them further to
CMP_TYPE_*

Refactored B*_MARKER to BIN*_MARKER

Refactored b* endian swappers to be*

Added size safeguards to writing fixed integers

Got rid of POSITIVE_FIXNUM_SIZE; it was something of a misnomer anyway

Got rid of NEGATIVE_FIXNUM_SIZE; it's never useful and therefore was
never used

Replace size_t with specifically-sized types where at all possible.

Start working on tests to add more than the MessagePack test suite as
it's incomplete; I need to add v5 tests and CMP-specific tests as well.