Expose packing and unpacking primitives

I had to write some `Packable` and `Unpackable` instances for my own data structure and for this found the primitives in the package very useful. Unfortunately, they are not exposed, so that I had to copy-pasted them into my project.

This pull request exports:
* `fromString`, `fromArray`, `fromPair`, `fromMap` from `Data.MessagePack.Unpack`
* `parseString`, `parseArray`, `parsePair`, `parseMap`, `parseUint16`, `parseUint32`, `parseUint64`, `parseInt8`, `parseInt16`, `parseInt32`, `parseInt64` from `Data.MessagePack.Pack`

It also adds Haddocks to all exported functions.