utf8 for gopher-lua

Implements Lua 5.3 utf8 for gopher-lua. To use, call

import (
	"github.com/PeerDB-io/gluautf8"
)

// add so that `local utf8 = require("utf8")` works
L.PreloadModule("utf8", gluautf8.Loader)

// or add to global env
L.Push(ls.NewFunction(gluautf8.Loader))
L.Call(0, 1)
L.Env.RawSetString("utf8", L.Get(-1))
L.Pop(1)