examples: Add memfs_ll.cc

examples: Add memfs_ll.cc

This is an initial implementation and quickly hacked together
within a few hours - issues expected.

This also increase to C++17 as memfs_ll makes use of more
recent features.

Background to create this was actually to be able to test large
file names (3 * 1024B), which couldn't be achieved with passthrough
file system as non of the underlying file systems seems to support
that.

Signed-off-by: Bernd Schubert <bschubert@ddn.com>