Replaced MetaLua parser with luacheck's own one.

Removed dependency on MetaLua.
   Now the only C dependecy is lfs, which is
   commonly preinstalled on Windows anyway.
Fixed compatibility with syntax added in Lua 5.2, Lua 5.3 and LuaJIT.
   Except \u{} escape sequences, see TODO.
Performance improved by a lot.
   Checking luarocks now takes 5x less time.

TODO:
   Write tests for lexer.
   Write tests for parser.
   Implement UTF escape sequences parsing(\u{}).
   Try to optimise lexer and parser even more. (See TODOs in source).
   Remove redundant optimisations.
      E.g. table field caching does not give much most likely.
   Repack lexer as table or repack parser as closure for consistency.
   See if trivial error message generation can be done without
      performance loss.
   Ensure global and lexer/parser related error propogation works.
   See if parts of local and goto resolution can be moved into parser.