Support timer queries for phases.

Support timer queries for phases.

This is useful for debugging slow chains; it can give information
about which phase takes the most time. Right now there seems to be
~5 ms in one of my test chains that disappear into nothing
(ie. show up in the fps counter with vsync off, but not in any
phase), but hopefully we can eventually solve that discrepancy.

Note that this is an ABI break.