tempfile module and shutil.rmtree fix

[BrianPugh]

shutil.rmtree had a reference to os.walk, which isn't in micropython. I updated it to use os.ilistdir and added unittests for it.

Initial implementation of the tempfile module. It currently has the following implemented:

• mkdtemp
• TemporaryDirectory

I only ever use TemporaryDirectory, and I assume others may have similar experience.

This is to make it much easier for me to update my pathlib unittests to be micropython-compatible.

[dpgeorge]

Thank you, these are good improvements and additions!

Merged in ee286ed through a99b801 with reworded commit messages.

I also added commit 56dc65b which adds manifest.py to `tempfile.