mir_build: Move and rename code for partitioning match candidates

[Zalathar]

I've always found it confusing that this code uses the word “sort” to describe partitioning candidates into buckets, since it isn't sorting them in the usual sense.

This PR therefore renames the relevant methods:

• sort_candidates → partition_candidates_into_buckets
• sort_candidate → choose_bucket_for_candidate

In addition, this PR moves those methods into their own buckets module, updates some comments to reflect the renaming, and also makes partition_candidates_into_buckets return a named struct instead of a tuple.

There should be no change to compiler behaviour.

[rustbot]

Some changes occurred in match lowering

cc @Nadrieril

[rustbot]

r? @saethlin

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