Changed timestamps from 64-bit to 32-bit.

SDL gives 32-bit timestamps, and 49 days oughtta be too long for
anyone to keep playing a video game.
Added `M_min()' macro for symmetry with `M_max()'
Added `mem', the memory management module.

The first memory management technique to be introduced is that of
memory regions.
Added thread safety and a global frame region

Now we have an agreed upon place for different parts of the system
to allocate memory that has a one-frame lifetime. The region will
be reset at the end of every frame.
Added `mem' module to the build system.

Also made a few minor changes so that it will build w/o warnings.