Please promote thread pools from experimental to stable

[joshtriplett]

I'd love to use the thread pool mechanism to have an application-wide thread pool sized based on the the number of CPUs, allowing multiple concurrent zstd compression jobs to share that pool rather than each having their own.

The thread pool API is currently in the experimental section. However, the API itself seems very simple: create a pool with a specified number of threads, free a pool, assign a pool to a compression context.

Would it be possible to promote those three functions, and associated opaque type, from experimental to stable?