Clean up terminology: step 1 — rename Region to Origin

[lqd]

A first step in cleaning up the Polonius terminology by:

• renaming regions into origins (except in datafrog closures and datalog rules comments, for now)
• whenever a single-letter generic type was used, using the full word instead
• assorted cleanups in comments or leftover random bits and bobs

[nikomatsakis]

@lqd did you plan to add a few more commits here?

[lqd]

@nikomatsakis not necessarily in this PR, I was thinking we could the renaming step by step, merging as each step is completed, what do you think ? (e.g. if others wanted to help, and so on)

But if you prefer a single big renaming-PR I can also do that.

[nikomatsakis]

step by step is better!