Auto merge of #7311 - ehuss:pipeline-timing, r=alexcrichton

Auto merge of #7311 - ehuss:pipeline-timing, r=alexcrichton

Experiment: Create timing report.

This is just an experiment, so I'm not sure if we'll want to merge it.

This adds an HTML report which gets saved to disk when the build is finished.  It is primarily geared for identifying slow dependencies, and for visualizing how pipelining affects the build.

Here's an example: https://ehuss.github.io/cargo-timing.html
You can mouse over the blocks to highlight the reverse-dependencies that are released when a unit finishes.  `syn` is a really good example.

It does a few other things, like displaying a message after each unit is finished.  See the docs for more information.