Support __pairs and __ipairs

Lua 5.2 introduces a nice feature where a metatable can override the
standard behavior of the pairs() and ipairs() functions. That would be
very useful in allowing a more standard syntax for our frame.args, and
it's very easy to do both in C and in Lua.

Change-Id: I37efc59a0c8876ee16184807e15fafbc07e2d288