Add pure variants of GetPropertyHelper all the way down
Carve out a safe path for ParGetPropertyCache

Only slot reads and nonexistent properties may be cached safely in parallel.
Change call-to-method-mut to an IC test
Two quick fixes to make Firefox local build stable.
Attempt to resolve crashiness by clearing pendingInvalidations more aggressively.
Merge branch 'make-firefox-stable' of github.com:pnkfelix/iontrail into make-firefox-stable