Shared memory synchronization primitives

[bavshin-f5]

To enable use of shared memory in the modules, we need a set of safe abstractions over src/os/unix/ngx_atomic.h, src/core/ngx_shmtx.h, src/core/ngx_slab.h and src/core/ngx_rwlock.h.

The goal is both to offer good safe Rust types and to be interoperable with the existing C types (upstream peers might be the most important here). It is acceptable if these goals lead to a two distinct sets of types.

Some inspiration could be taken from the Rust for Linux project and the Rust Atomics and Locks book.