Add a wrapper transform that runs a child transform in a separate process

This adds a transform that transfers data to/from a worker process, and the worker process runs a transform on the templates it receives. The point of this is of course to isolate transforms that aren't thread-safe, so even if we can't use multi-threading directly, we can wrap those transforms in single thread processes, and avoid problems that way.

The IPC aspect seems to be working reasonably well, I guess the open question is if we should use br as the worker process (which require exposing some internals in a not necessarily great way), or make a dedicated application for that.