Version core runtime

[petervdonovan]

See #342 (comment).

This is low-priority.

We can start out by just providing a SHA so that we have a quick way of making absolutely sure that the core runtime is compatible with the RTI or with any plugins such as the tracing plugin. There are automated ways in CMake to inject the SHA into the source code, so this should be easy. Additionally, as Christian pointed out, using SHAs is compatible with also using another versioning scheme like semver -- the core runtime can accept either and check either against itself.

In the near term I would like to avoid arguments about semver and about which semvers should align with which and which things should be independently semver'd and when is a good time to start using semver and so forth. If this issue is resolved in the near term I would prefer it to be a targeted effort to resolve a current pain point which does not have to set any unbreakable precedent, and it should probably use a SHA and not a semver.