Provide a way to compute full digests of projects

[oyvindberg]

If you have many deployables it's incredibly useful to be able to calculate which sets of deployables have changed between arbitrary git commits. I think bleep can provide a really simple and powerful approach for this:

Since the build is just data, it follows that we can digest each project in the build, along with its transitive dependencies. We also have all the source code, so we can include those too in the digest. We can likely ask git for checksums to make this a really cheap operation.

By versioning each project with that digest, it's suddenly trivial to figure out everything that has changed between and will need a new deploy! super easy, and super powerful!