Preprocess class styles in cascade

Continuing along the lines of #831, this moves more preprocessing of style
declarations into cascade() from cascadeClasses(), to avoid redoing this work
each time a class is changed. On my Mac, this gets me another 3-6x speed
improvement: cascadeClasses() is down to < 15ms for me in most situations.

These changes moderately slow down cascade(), since all layer paint properties
are preprocessed (vs. just currently active classes), but hopefully a small
delay is tolerable on changing the map stylesheet, and is a small price to pay
for very fast subsequent class changes.

Also fixes #789.