Share profile loggers between nested evaluations

[lihaoyi]

We broaden the scope of profileLogger/chromeProfileLogger from within def evaluate to instead last as long as the Evaluator itself, and share it with any sub-Evaluators created e.g. to redirect output during show

Evaluator is now AutoCloseable, and needs to be closed after being used in MillBuildBootstrap and in UnitTester

For now I decided not to share the profilers between different evaluation levels of the meta-build, since they already don't overlap: one goes to out/, the parent to out/mill-build/, etc. We could do that if we think it would be useful to give people a consolidated profile, but it might be a bit more work (e.g. we would need to properly prefix entries in the profile to distinguish the different meta levels).

Added a unit test to ensure the show compile mill-profile and mill-chrome-profile contain the expected keys for both show and for compile and some of its upstream tasks

Fixes #3879

[lefou]

Looks good to me.