[MERGE] redo boolean context

Overhaul the stuff that flags an op as being in boolean context (currently
just padhv and rv2hv). Make the mechanism in rpeep() general, so that
other ops can be easily added in future, and add a generic testing
framework for such ops in t/perf/optree.t.

This alters the amount on situations recognised as being in boolean
context (mainly increasing them).