Expose more methods from mapped guards

[rybot666]

While very useful, mapped guards do not currently expose methods such as raw and force_unlock, which makes using them in certain contexts quite difficult. From what I can tell, there's no soundness reason why they haven't been exposed. Would a PR adding the same APIs to them as the other guards be accepted?

[Amanieu]

Yes, I would I accept a PR for these. Fundamentally these are unsafe functions so it's fine to expose them with appropriate warnings.

[Earthcomputer]

I also came across a situation where I needed to downgrade a write mapped guard to a read mapped guard. So there are a few APIs missing.