Add MessagePack support

[jarmuszz]

This ticket will be the root of the MessagePack implementation process.

Similarly to fs2-data-cbor, the fs2-data-msgpack should be split into two modules:

• The low module which will work on a flat model that closely mimics the raw MessagePack format. Data in this model will not be converted into language-level values but rather kept in it's byte form.
• The high module that will work on an AST typed accordingly to the implementation specification. The parsing and serialization in this module should most likely be built on top of the low module.

We can split the process of implementing the format into the following parts:

• [ ] low
  • [x] model and parser (#604)
  • [X] msgpack encoder (#628)
  • [X] validator (#628)
  • [ ] documentation
• [ ] high
  • [ ] model and parser
  • [ ] msgpack encoder
  • [ ] documentation