worker: support more return types in posix worker This allows us to return things that aren't tuple(int, str) from worker functions. I wanted to use marshal instead of pickle, but it seems to read from the pipe in non-blocking mode, which means it stops before it sees the results. The windows worker already supports arbitrary return values without serialization, because it uses threads instead of subprocesses. Differential Revision: https://phab.mercurial-scm.org/D3845