Laziness in the compiler

[scottmcm]

@CAD97

While clean builds are still important, what really matters the most is low-to-medium optimization incremental builds, since that's what bottlenecks development iteration time.

And I think the biggest improvements here would need an even more drastic architectural change. Loading all the incremental compilation data is itself a huge cost, so really fast incremental might require doing something like daemonizing the compiler, similar to how R-A is a process that just stays running.