Add Continuous Integration

[Schuyler Eldridge]

Add some CI. Any vendor should be fine.

There is a question of what should be checked in CI. The following may be useful:

• Current Chisel release version
• Published snapshots
• Master

As some of these may move independent of this repository, these would likely benefit from nightly regressions in addition to PR-gating regressions.

[Kevin Laeufer]

fixed