File system `mount`

[samuelcolvin]

We need to support mounting one or more file system directories into monty.

We need to be extremely careful that monty code cannot follow or create symlinks or perform path traversal and in any way read or write outside the given directory.

This should be implemented in Rust so

1. it works in python and js easily
2. we can be as explicit as possible about what is and is not allowed
3. it can be added to the CLI making it much more powerful

This is effectively a rust implementation of the OSCall functions, using a real filesystem.

As with just-bash, we should support modes:

• read/write
• read-only
• maybe an "overlay" mode where writes are handled separately, but make this isn't required?