Add GitHub Actions to build installation packages for develop.

This workflow builds installations for MacOS, Windows, and Ubuntu from
the tip of the develop branch.

Author: Mark R Tuttle

.github/workflows/README.md

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+# CBMC packages
+
+This project builds installation packages for the tip of the develop
+branch for MacOS, Windows, and Ubuntu.
+
+There exist installation packages for the latest stable releases of
+CBMC on MacOS and Ubuntu.
+
+On MacOS:
+* brew install cbmc
+
+On Ubuntu:
+* sudo apt-get install software-properties-common
+* sudo add-apt-repository ppa:mt-debian/cbmc-backports
+* sudo apt-get update
+* sudo apt-get install cbmc
+
+This project uses GitHub Actions to build installation packages for
+the tip of the develop branch for MacOS, Windows, and Ubuntu each time
+new commits is added to develop.  The packages reside on GitHub as
+artifacts that can be listed using the GitHub Actions API.
+
+A separate project implements a web page hosted on GitHub Pages that makes
+it easy to find the installation package for the tip of develop.
+
+The stable installation packages describe above for MacOS and Ubuntu
+install into the local operating system's equivalent of
+/usr/local/bin.
+This project builds two kinds of packages:
+* cbmc installs into the equvalent of /usr/local/bin
+* cbmc-latest installs into the equivalent of /usr/local/cbmc-latest/bin,
+  and makes it possible to have two copies of cbmc --- a stable release
+  and a tip of develop --- side-by-side on the same machine.
+
+For each operatin system:
+* The MacOS package is just a tar file of a directory containing the
+  binaries. The directory should be unpacked and placed in the search
+  path. Using Homebrew, "brew install cbmc" will install the latest
+  stable release.  These tar files are intended only to distribute the
+  development versions between stable releases (Homebrew repository
+  updates of the stable versions are quick).
+
+* The Windows package is an Microsoft Installer (msi) for Windows 10
+  with Visual Studio 2019.  It can be installed by double-clicking on the
+  installer or runnin `msexec /i <filename>`.
+
+* The Ubuntu package is a Debian package that can be installed with
+  `dpkg -i <filename>`. There are packages for Ubuntu 18 and Ubuntu 16.
+  These packages are intended to distribute the development versions
+  between stable releases, but also to produce the stable packages uploaded
+  to a Debian or Ubuntu PPA.
+
+The file packages.yaml defines the workflow for GitHub Actions to build the
+packages. Each package is defined by a job that runs in its own
+container. The subdirectories contain files and data needed to build
+each of the packages.

.github/workflows/TODO.md

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+Left to do:
+* Make the Debian builds run the full debian package flow.
+* Refactor the workflow to invoke a makefile that can be invoked either from
+  the workflow or from a command line to build the package.
+* Make the workflow distinguish between stable versions and latest versions
+  Stable versions should write into cbmc (or bin) and latest versions
+  should write into cbmc-latest (so they can exist side-by-side).  Windows
+  packages will have to have different product guids for stable and latest.
+* Transform version numbers from 5.12 to 5.12.0.id where id is some value
+  that increments with each build.  A good candidate is `github.run_id` or
+  `github.run_num`.
+* Can we automate the generation of repository packages (eg, homebrew)?
+
+Plans:
+* Wait for Michael's Debian builds and fit them into the workflow
+* Refactor the workflow to invoke Makefiles
+* Prepare PR
+* Do remaining items as PR refinements.