[Feature Request] Basic build caching on CI

[thedavidharris]

CircleCI has some cache support that should let us cache the artifacts to some extent. It's pretty easy to add in, but it depends on the flow you want to use:

1. Cache artifacts on the master branch always with a key

or

2. Use the master cache, then cache artifacts on each branch after a PR is made and built under a separate key

As long as CircleCI is running tests on the "default" branch each time 1 isn't too bad alone and should give most of the benefits