We should allow clients to overriding classpath hashing

[romanowski]

Zinc has a lot hooks to improve incremental compilation performance in case of really big projects. However when we hash classpath we just do tons of IO operations without any way to e.g. reuse libs hashed before.

In case of 100+ projects with 100+ libs and on slower IO (e.g. windows) it takes a while during each compilation (even no-op ones).

I haven't found a way to do it without breaking binary compatibility but simple proposal how it can be done is here: https://github.com/romanowski/zinc/tree/hashing-classpath-proposal

@stuhood I think pants may also benefit from caching classpath hashes.

[jvican]

Related to #371.

I agree this is a legit use case.