Use fast hashing functions with IndexMap and IndexSet everywhere.

[jimblandy]

Various spots in WGSL are using plain IndexMap and IndexSet from the indexmap crate, rather than a variant that uses the same faster hash function as the rest of Naga. Change these sites to all use the faster hash function.

[ErichDonGubler]

LGTM from a code perspective, but I have a question that I think needs to be answered before merging: where are the benchmarks? How can I be sure this is actually making things faster?

[jimblandy]

My intent here is not actually speed, but consistency. We have FastHashMap and FastHashSet, so it's odd to randomly use other hash functions depending on who wrote the code.

[jimblandy]

LGTM from a code perspective, but I have a question that I think needs to be answered before merging: where are the benchmarks? How can I be sure this is actually making things faster?

This is a good point. Filed #2499 (and naturally #2498).