Implement per-target fingerprints

This commit refines the granularity of fingerprints from packages to targets,
building on the existing infrastructure. All invocations of rustc now include
the --dep-info flag which will emit makefile-like dependency information. This
information is then parsed later on to determine if all the files are fresh.

The purpose of this commit is to refine the set of files which indicate that a
target needs to be re-built. Before this commit if you modified a test it would
rebuild the entire library, but after this commit it only rebuilds the relevant
test.

Closes #214
Closes #289