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/reset-brew.sh

@@ -0,0 +1,34 @@
+#!/bin/bash
+
+# Copyright (c) 2019 Kevin R Croft <krcroft@gmail.com>
+# SPDX-License-Identifier: GPL-2.0-or-later
+
+# This script removes all existing brew packages, and then
+# re-installs some important basic packages (listed below).
+#
+# Usage: ./reset-brew.sh
+#
+set -xuo pipefail
+set +e
+
+# Pre-cleanup size
+sudo du -sch /usr/local 2> /dev/null
+
+# Uninstall all packages
+# shellcheck disable=SC2046
+brew remove --force $(brew list) --ignore-dependencies
+# shellcheck disable=SC2046
+brew cask remove --force $(brew cask list)
+
+# Reinstall important packages
+brew install git git-lfs python curl wget jq binutils zstd gnu-tar
+
+# Clean the brew cache
+rm -rf "$(brew --cache)"
+
+# Post-clean up size
+sudo du -sch /usr/local 2> /dev/null
+
+# This script is best-effort, so always return success
+exit 0
+