fix "illegal invocation" errors for unbound perf.now calls in chrome

There's certainly a cleaner way to do this, but I am lazy and this works.
Merge branch 'patch-2' of https://github.com/michaelficarra/purescript-signal into michaelficarra-patch-2

Conflicts:
	src/Signal/Time.purs
Merge branch 'michaelficarra-patch-2'