Allow unsafe creation of new lock guards

[Lochlanna]

This PR changes the visibility of the functions that produce the lock guards to public.
The inspiration for this comes from my desire to extend the mutex with new locking functions (async locking for example).

I was initially concerned that exposing this might be a bad idea even if it is useful. However, I decided that exposing these functions was no worse than the already public force_unlock functions.

[Amanieu]

Could you rename these functions to make_guard_unchecked so that it's clear that these functions are unsafe and that you should carefully read the documentation for them?

[Amanieu]

This is missing methods for other guard types such as upgradable guards and Arc guards.

[Amanieu]

Slight nit: could you rename the arc versions to make_arc(_read/write/upgradable)_guard?

After that it should be good to merge and I will publish a release to address #406

[Amanieu]

Thanks!