Improve Rust support

[antoinevg]

The Rust support is currently heavily orientated towards cynthion.git usage. This can be greatly improved without too much work.

• Move the zero-dependency Rust example from the cynthion.git repository here.
• Move lunasoc-pac and lunasoc-hal from the cynthion.git repository here.
• Extend lunasoc-pac and lunasoc-hal to work with arbitrary lunasoc designs.

Note: some of this overlaps with upstream amaranth-soc goals and it may turn out that we can even deprecate our own work in this area.