also delete mkrw.go, we can maintain this file by hand.
detected by go-perfguard
move cqcode escape&unescape to new package
encoding/gob
Two benefit below: * shrink go-cqhttp binary size about 200KiB * shrink database file from 2.8M to 1.56M compared with v2 database Also provide a tool to migrate v2 database: https://github.com/RomiChan/gocq-leveldb-migrate