Draft/WIP.

Haven't given much thought to code size (especially when the feature is disabled). The allow_frozen flag to the stat_ methods in particular needs some thought.

I considered a few other ways of implementing this, in particular making it detect the filesystem version of the file while searching sys.path but this creates a complicated interaction with things like loading the parent modules. Instead this approach says "a frozen file can be overridden by a filesystem file" rather than interfering with file discovery.

Filesystem means "VFS", so note this would work with mpremote mount too... (perhaps via #8914 we could have a way to make mpremote construct this "files that have changed relative to freezing" directory to mount)

This allows frozen files to be overridden by filesystem .py or .mpy files from a specific path specified in micropython.frozen_overlay.

The idea is that during development, a whole directory tree might be frozen
but for fast testing iteration it can be useful to just replace a couple
of files. This allows the same directory structure to be defined, but any
time a file is about to be loaded from frozen, the same path is attempted
in the overlay path.

Note: the overlay path should not be part of sys.path, otherwise it will
prevent the frozen path from being found.