Releasing and tagging

[palmskog]

To make the library easier to use for others, one option is to do explicit tags/releases in the repo that are guaranteed to work with a certain combination of versions of operating system + compiler + HOL4. Continuous integration and other tooling (discussed in #99) can be used to support generating these tags/releases.

Although always relying on HEAD of project's repo (like Google does internally) avoids problems with maintaining backwards compatibility, it typically comes at the cost of user flexibility.

One option for release naming is to use semantic versioning.