Add Lua "snogray.vector" module

This is a thin wrapper around C++ std::vector, with support for float,
int, and unsigned element types.

It is slightly faster than a Lua table, but more importantly, uses
much less memory and is cheaper to pass to C++ functions that already
take a std::vector argument.
Handle snogray.vector arguments in SWIG
Add "resize" method to snogray.vector
Add "reserve" method to snogray.vector
Add "clear" method to snogray.vector