Currently, every iteration of the main loop in `generic_extension`
instantiates a `Parser`, which is expensive because `Parser` is a large
type. Many of those instantiations are only used immutably, particularly
for simple-but-repetitive macros of the sort seen in `html5ever` and PR
68836.
This commit initializes a single "base" parser outside the loop, and
then uses `Cow` to avoid cloning it except for the mutating iterations.
This speeds up `html5ever` runs by up to 15%.
