Cleaning up old PRs, branches, and issues

Emscripten has a lot of old PRs, branches, and issues.

These are problematic for several reasons:

1. Branches show up in all developers git checkout. If there are 300+ branches it can be confusing and make it hard to know which ones to pay attention to.
2. Bug triage and PR review are hard when there are so many open. This is broken windows syndrome where the existence of so many open issues makes it hard to focus on the new/important ones and know where to start.

I propose that we try to slowing burn down the open bugs/branches/PRs, starting with PRs. I've been using simple search for least-recently-updated PRs and then trying to process 10 or 20 each day: https://github.com/emscripten-core/emscripten/pulls?q=is%3Apr+is%3Aopen+sort%3Aupdated-asc.

If folks have any other ideas for how to burn down this list please suggest them here.