implement new low-level and high-level mutexes and condition variables

[thestinger]

This should be a pair of types built directly on the platform primitives. On Windows, it should directly wrap CRITICAL_SECTION and CONDITION_VARIABLE while on *nix it would wrap pthread_mutex_t and pthread_cond_t. It can then be wrapped into a new high-level safe API.

C++11 also exposes a portable recursive mutex, which would be an easy addition. It could allow expose adaptive mutexes (faster, at the expense of fairness) when available since falling back to normal mutexes is a portable implementation.

[thestinger]

It should provide the full set of features including timed locks and timed waits.

[gamazeps]

@thestinger Is this a 'simple' ffi you are interested in ?

Also, why use an ffi to pthread instead of implementing it in Rust as #17094 suggests ?