This PR overhauls the CI into something hopefully a bit simpler/sensible. Now, we have recategorized the github actions into just two workflows: long and short tests. The short tests should provide quick feedback if there's some typo/build error.

All per-release tests on Travis have been changed to per-PR, and have either stayed in Travis or been moved to GH-actions if possible. Those remaining in travis typically are the more niche build configurations and whatnot. Adding a nightly build to travis as discussed offline isn't necessary since we can actually run these per-PR, since the remaining tests aren't so long (and Travis runs them in parallel, and completes faster than dev-long-tests).

Note that no tests have been removed, and tests run either as frequently or more frequently than before, though they may have been migrated or broken up into independent tests.

So, this PR:

• Removes all existing gh actions workflows.
• Adds dev-short-tests and dev-long-tests workflows. Short tests are defined as those that typically finish under 5min, long tests are typically more than 10 min.
• Migrates as many of the travis and appveyor tests to gh actions.
• Breaks up some of the really long tests that were basically multiple tests into individual jobs. Now the max length test is <40min (instead of 1hr+).
• Adds memory sanitizer to oss-fuzz test.
• Adds build-cancellation to redundant GH actions builds, to prevent long queues.

Next steps:

• Migrate even more off of AppVeyor.

Open questions:

• Should we still support visual studio 2013 and less? According to microsoft docs the 2013 visual studio "RTW" and below are no longer supported.
• Should regressiontest get added as per-PR? I don't see why it'd make more sense to run nightly rather than per-PR. It's also not that long.
• What to do regarding the newest clang static analyzer warnings? It seems like most of these are false positives.