Add complete sext decode nif

Expanded the existing binary decoding nif to decode an entire term, or
at least the data types we care about. Floats, ints beyond 64 bits and
bitstrings are not handled. Unhandled data types make the decode NIF
return an error code that will cause the erlang decode to be used
instead for safety and to use the existing test suite.

All eunit tests were passing.