cache chalk queries

cache chalk queries

This gives a significant speedup, because chalk will call these
functions several times even withing a single revision. The only
significant one here is `impl_data`, but I figured it might be good to
cache others just for consistency.

The results I get are:

Before:

from scratch:   16.081457952s
no change:      15.846493ms
trivial change: 352.95592ms
comment change: 361.998408ms
const change:   457.629212ms

After:

from scratch:   14.910610278s
no change:      14.934647ms
trivial change: 85.633023ms
comment change: 96.433023ms
const change:   171.543296ms

Seems like a nice win!