Add an option to output crate build timings

[luser]

Currently cargo outputs a line at the end of the build like Finished dev [unoptimized + debuginfo] target(s) in 4.78 secs giving the overall build time. It would be nice if there was a way to get out build times for the individual crates that were built, and even timings for the individual build steps within each crate, like compiling a build script, executing a build script, compiling the crate, compiling any binaries, etc. The most useful thing to me would be to just output a JSON file, probably.

[federicomenaquintero]

Note that with #7405 this is more or less already in place.

[ehuss]

Yea, I'll close in favor of #7405.