Improve runtime hashing performance

[udoprog]

This improves the performance of common runtime hashing operations.

Improves the performance of the aoc_2020_1b benchmark by ~10% since this utilizes instance fns heavily.

Before:

test aoc_2020_1b ... bench:     668,483 ns/iter (+/- 20,836)

After:

test aoc_2020_1b ... bench:     582,291 ns/iter (+/- 12,307)

Thanks to @tgolsson for highlighting this!