This commit adds a call to `pthread_setname_np` in
`alien_worker::spawn`, so each alien worker thread receives a
descriptive name. This makes debugging, monitoring, and performance
analysis easier by allowing alien workers to be clearly identified
in tools such as `perf`.
Introduces a util which launches a new OS thread and accepts
callables for concurrent execution.
Meant to be created once at startup and used until shutdown,
for running nonpreemptible, 3rd party, non-interactive code.
Note: this new utility is almost identical to wasm::alien_thread_runner.
Maybe we should unify them.
