bigint lua port

[Ted Unangst <ted.unangst@...>]

I ported (as in rewrote) a C bigint library to Lua.  It only covers
the basics, but it's written in pure Lua, no C binding, and it's
fairly quick.  I figured it may be useful to someone if you want to
avoid external dependencies.  I think it works, but was still fixing
bugs recently, so user beware.

Lua code here: http://bitbucket.org/tedu/bigintlua/src/tip/bigint.lua

C original from here: http://www.acme.com/software/bigint/