Wrappers for golang io

NOTE: These aren't exposed to LUA directly, but used as utilities for other classes, which need the bridge between lua file and these interfaces:

• io.Reader
• io.Writer

See usages of CheckIOReader and CheckIOWriter in json and yaml modules to treat args as the appropriate go type.

Going the other way, ReaderFuncTable and WriterFuncTable are provided for libraries that need to register a type that behaves like a io.Reader or io.Writer and may used as a file from lua. See example uses of this in the strings and base64 modules, such as this snippet from base64.

//registerBase64Encoder Registers the encoder type and its methods
func registerBase64Encoder(L *lua.LState) {
	mt := L.NewTypeMetatable(base64EncoderType)
	L.SetGlobal(base64EncoderType, mt)
	L.SetField(mt, "__index", lio.WriterFuncTable(L))
}

//registerBase64Decoder Registers the decoder type and its methods
func registerBase64Decoder(L *lua.LState) {
	mt := L.NewTypeMetatable(base64DecoderType)
	L.SetGlobal(base64DecoderType, mt)
	L.SetField(mt, "__index", lio.ReaderFuncTable(L))
}