Cirrus CI and macOS on M1 CI

[rptb1]

Adding Cirrus CI to replace Travis CI.

Adding CI build and test on macOS on M1 (XCA6LL), which will allow us to test, review, merge pending A6 pull requests.

Fixes #109 .

Creates new document design.mps.ci.

[rptb1]

I asked some questions of the Cirrus CI community at cirruslabs/cirrus-ci-docs#1147

[rptb1]

Getting there...

[rptb1]

Submitted a request to Cirrus CI to improve feedback. See cirruslabs/cirrus-ci-docs#1149

[rptb1]

Unfortunately, there don't seem to be FreeBSD on arm64 images available. There's nothing but amd64.

gcloud compute images list --project freebsd-org-cloud-dev --no-standard-images | grep -v amd64

[rptb1]

It appears (from the links in GitHub's checks list on this pull request) that Cirrus CI is running on the underlying branch, not on a trial merge between the branch and master. That's inconsistent (and less useful) than GitHub CI and Travis CI.

[rptb1]

At this point we have four CI systems on the go:

1. GitHub for I6 except FRI6*: default, most amd64 platforms except FreeBSD, including macOS, plus misc other checks
2. Cirrus CI for XCA6LL, LIA6*, FRI6*: macOS on Apple Silicon, Linux on arm64, FreeBSD on amd64.
3. Travis CI for FRA6*: FreeBSD on arm64
4. Read the Docs: build and publish the manual

So this branch should really pop design.tests.ci into its own document that explains all of the above and allows it to be maintained.

[rptb1]

It appears (from the links in GitHub's checks list on this pull request) that Cirrus CI is running on the underlying branch, not on a trial merge between the branch and master. That's inconsistent (and less useful) than GitHub CI and Travis CI.

Submitted a query about this to Cirrus CI discussions.

[rptb1]

Proof of concept achieved.