msgpack encoding library for gopher-lua in PeerDB

This follows many ideas from gluajson

For now there's no unmark function, as most marker types do not have a clear & precise Lua value

We also don't have a ubiquitous MarshalJSON trait to rely on, instead custom encodings must be done with a Packer interface:

type Packer interface {
  PackMsg([]byte) []byte
}

Which is passed the entire msgpack buffer & should append accordingly, returning the result

---

decode is not currently implemented, only encode

Lua strings are checked, if valid utf8 they are encoded as str, otherwise as bin

encode checks UserData for a __msgpack metamethod, the result of which is encoded

If no __msgpack metamethod exists, the Value is checked. If it implements the Packer interface, then it is invoked. Otherwise the following types have predictable implementations:

• string (not checked for valid utf8)
• []byte
• uint64
• int64
• time.Time

There exists the following marker methods:

• raw takes a string
• array, map takes a table (useful for encoding empty arrays)
• bin, str takes a string
• signed, unsigned takes a number, or parses string
• f32, f64 takes a number
• time, time32, time64, time96 takes number representing time since unix epoch in seconds, or string with optional format (default RFC3339), or UserData with time.Time value
• ext takes a number for type & string for bytes