Build caching

[MaddieM4]

Some sort of data storage, with the particulars intentionally encapsulated by the backend, for recording that a particular pure transformation on archive X will result in archive Y. This allows for rapidly fast-forwarding through build steps where we know what the outcome must be.

Some build steps are not, on their own, pure. For example, importing and exporting (and cmd-impure). But the fact that impure build steps produce archives (which are now immutable going forward and might happen to be referenced in the cache) allows the cache mechanism to pick back up after impure steps. It will be awhile before cmd (the pure variety happening in a sandbox) is properly designed and implemented, but in the meantime, we should be able to at least get pretty good build performance in the short term with cmd-impure thanks to the cache resume properties.