Create a program to handle release chores

There's a few tedious and mechanical steps for our release process around replacing strings and updating docs. See RELEASING.md for all the steps.

It should be relatively easy to create a program that does the various string processing. The basic logic it needs to do is:

• Update CHANGELOG.md
  • Replace the "Unreleased" title with [X.Y.Z] - YYYY-MM-DD
  • Replace 0.0.0 with X.Y.Z
  • Replace v0-0-0 with vX-Y-Z
  • Caveat: don't modify the "unreleased template" that is commented out.
• Replace VERSION_NEXT_* markers
  • caveat: don't replace them in CONTRIBUTING.md, RELEASING.md, and .* dirs (.githhub etc)

Something more advanced could look at which VERSION_NEXT_{FEATURE,PATCH} markers exist to figure out the next semantic version, but that's just a nice to have.

Just being able to do tools/private/release.py 1.4.0 would be a big improvement over all the ad-hoc stuff we have to do today.