bake: Make pipeline processing multi-process. Not many changes here, as it's pretty straightforward, but an API change for processors so they know if they're being initialized/disposed from the main process or from one of the workers. This makes it possible to do global stuff that has side-effects (e.g. create a directory) vs. doing in-memory stuff.

import os


def slashfix(path):
    if path is None:
        return None
    if isinstance(path, str):
        return path.replace('/', os.sep)
    fixed = []
    for p in path:
        fixed.append(p.replace('/', os.sep))
    return fixed