CI: Now using Merge Queue

[bluss]

Suggested by @adamreichold.

We have github actions checks that check PRs - we run all tests in various toolchains, and then we have extra checks - cross compiled tests and cargo-careful for now, and those slower tests only run in the merge queue.

When a PR is reviewed and tests are passing, it can be accepted for integration. It gets put on the merge queue. The PR is merged to a test branch and then tested with the complete set of tests and checks, then automatically integrated into the master branch.

There are some quirks to the github merge queue, we had to add the conclusion job that just gathers status of all other tests. (Reason: required check on PR has to have the same name as the required check in the merge queue, and additionally we don't want to list 5 or more different items in the required checks list.)

When a PR is finished it also shows the summary just of the on-PR check and not from the checks done in the merge queue, which is a bit surprising - however, if the merge check fails, a clear status about this will be visible on the PR. The merge queue jobs are visible in the Actions tab as usual. The merge queue UI is here but it doesn't seem to have any fun features right now, doesn't tell us much: https://github.com/rust-ndarray/ndarray/queue/master