This is a fix for https://github.com/micropython/micropython/issues/2209:
by default a file created using open() uses text translation mode so writing
\n to it will result in the file having \r\n. This is obviously problematic
for binary .mpy files, so provide functions for setting the open mode
and use binary mode in mpy-cross' main().
.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.
