use __index metamethod in ipairs

[matthias314]

With this PR, GopherLua respects the __index metamethod in ipairs. This change was introduced in Lua 5.3. (Lua 5.2 had an __ipairs metamethod, which was deprecated in 5.3.)

This would be particularly useful in the context of gopher-luar (CC @layeh). To iterate over a Go array or slice t from Lua, one currently has to access the iterator via t() (__call metamethod). This means that one has to write separate code for iterating over Lua lists on the one hand and Go arrays/slices on the other. With this PR, the same code would work for both cases.

Performance: On my machine, summing up the values of an honest Lua list with one million integer entries becomes about 15% slower with this PR. For non-trivial loop bodies the slow-down would obviously be smaller.

EDIT: Iterating over a Go array/slice doesn't work yet because gopher-luar doesn't return nil if one tries to access a non-existing entry, but instead errors. Iterating over a Go map with indices 1, 2, ... does work.