Refactor and expand the CI workflows

- For each OS, builds of the default compiler plus the
  latest-supported compilers are run. When multiple operating systems
  are supported (such as Ubuntu 16.04 and latest), a build on
  the oldest OS using its default compiler is also performed.

- Debug builds are used because they often are more thorough at
  detecting coding issues (debug warning counts are higher).

- Runtime dynamic sanitizers are added and serialized per-compiler.
  Their build and runtime log-files are xz-compressed, and then
  GitHub's asset upload Zips the log directory.

- Each workflow now holds the maximum allowed compiler warnings
  per-build, so we can have tighter control of when new warnings
  are introduced (that would otherwise pass if still below the
  maximum)

- Use of github's new 'cache' feature has been leveraged to restore
  the brew, macports, and msys2 environments to eliminate the
  lenghthy setup times for those environments.  If a new cache
  is needed, then we simply increment the cache `key:` value and
  the next CI run will archive new caches. (Note that GitHub has a
  400MB limit on cache size however they have already said they
  are raising it - so we might be able to cache out longest running
  job which is MSYS+Clang)

- Where it makes sense, multi-line workflow statements have been
  broken out into .github/scripts as files to make the workflow YAML
  leaner and more readable, while giving us a richer environment in
  the scripts.

.github/scripts/tar

@@ -0,0 +1,19 @@
+#!/bin/bash
+set -euo pipefail
+rcode=0
+# If we're running outside of GitHub then run tar normally
+if [[ -z "${GITHUB_WORKFLOW:-}" ]]; then
+	"/usr/local/bin/gtar" "$@"
+	rcode="$?"
+# Otherwise use zstd and ignore tar's return-code
+else
+	first_arg="${1/z/}"
+	shift
+	[[ "${first_arg}" == "-x" ]] && decomp="-d" || decomp=""
+	set +e -x
+	"/usr/local/bin/gtar" \
+		--warning=none \
+		--use-compress-program="zstd -T0 -19 --long=31 --no-progress -v ${decomp}" \
+		"${first_arg}" "$@"
+fi
+exit "${rcode}"