Expose more packet encodes/decoders to Lua

Currently, the mysql.proto package exposes some encoders/decoders to Lua, mostly simple ones to deal with OK/ERR and auth/auth_response packets.

Despite this, network-mysqld-proto.c has more available, including those for result set packets, including the 4.1 packet decoders containing column type information.
For development and debugging purposes, it is interesting to see exactly what type is coming across the wire. The wireshark MySQL dissector currently does not provide this information, and Proxy currently doesn't help much, either (unless we poke around the raw packet manually, which is error-prone and highly irritating).

The goal of this project is to provide a complete interface to all available packet encoders/decoders and come up with some sensible Lua interface to these. A start is available in lib/mysql-proto.c.