Think comprehensively about system utility APIs

This is some feedback I've heard half a dozen times this week at Python Brasil: Python's system utility APIs are in too many places, and do not have any sort of conceptual integrity, to wit: `os`, `os.path`, `pathlib`, `shutil`, `subprocess`, `fileinput`, `stat`, `glob`, `platform`, `errno` (to say nothing of their 3rd party alternatives) all appear to perform a partial, confusing, and inconsisten portion of what we might think of as system-scripting utilities.

Some specific ideas:
- `Path` object abstraction from the beginning. _All_ path operations go through it, no ridiculous family of functions that take strings.
- One process spawning API, no `fork()`/`exec`.