Track build/test times of individual crates across run.

[tomprince]

The current estimation of time is based on taking the average time for the crates that have been run in the current process. Since crates can vary significantly in how long they take to build (or fail to build), the estimates can vary wildly (I have seen variations of at least 2x). This could be improved by averaging over previous runs of the remaining crates, perhaps adjusted based on the current run of the already completed crates.