clean up old files

Stream: general

[Reid Barton]

I wrote two Python scripts this morning to help with the transition of moving everything to src/. One deletes .olean files without a corresponding .lean file, the other deletes directory trees that contain no files.
I notice @Simon Hudon also wrote an equivalent shell script in the fold PR (https://github.com/leanprover/mathlib/pull/376/files#diff-81a7c4de26bf82dac09e943b11b95792).
Windows users: is one of shell scripts/Python scripts more usable than the other?

[Jeremy Avigad]

BTW, I simply used git clean -xdf to do this (git clean -xdn will do a dry run without deleting anything).