rtt_roscomm: Cache typekits in repository

[jbohren]

Currently, when you make a new workspace, or re-build a workspace, all of the generated typekit code has to be re-generated from scratch. This is despite the fact that many message definitions rarely change.

A lot of build time could be saved by caching the .msg and .srv files in the checked-in code, and only re-generating the code if their checksums differ.