support for str format family

[vbmithr]

Currently, only "raw", i.e. bin format family is supported.
I'd like to contribute but have to admit that I'm a bit afraid by the code: did you really extract this library from a Coq proof :) ?

[mzp]

I appreciate your help.

I extract https://github.com/msgpack/msgpack-ocaml/blob/master/lib/core/msgpackCore.ml from Coq proof, and write other codes by hands.