.mpy files contain the name of the source file that they were compiled
from. This patch adds a way to change this name to an arbitrary string,
specified on the command line with the -s option. The default is to use
the full name of the input filename.
This new -s option is useful to strip off a leading directory name so
that mpy-tool.py can freeze packages.
Building in 32-bit mode was only to reduce binary size on 64-bit machines
and is otherwise not needed. Having it forced to 32-bit meant an
unnecessary dependency on 32-bit libraries that is now removed.
