We ripped that out when initially porting VfsPosix to work on ESP32.
This re-introduces it.

Fixes #10.

Previously MicroPython would mount LittleFS directly from the partition
by itself, using its own VFS stack.

This reconfigured the VFS layer in MicroPython to use POSIX
open/read/write/etc calls, which are backed by ESP-IDF. Now, instead of
managing (possibly multuple) block devices at different mountpoints,
MicroPython simply passes through all VFS access to the ESP-IDF VFS.

Then, we make the ESP-IDF VFS mount a wear-leveled FAT (backed by the
same partition which previously held LittleFS) onto /.