[naga] Use `HandleSet` instead of `BitSet` where appropriate

[jimblandy]

Use the more strictly-typed HandleSet for sets of arena handles, rather than the untyped BitSet.

This depends on #5862, so it's marked as a draft PR for now.

• [naga] Move HandleSet from compact into arena.

• [naga] Use HandleSet in naga::proc::index.

  Change naga::proc::index::find_checked_indexes to return a HandleSet<Expression>, rather than an untyped BitSet.

  Fix uses in the Metal Shading Language backend.

• [naga] Change HandleSet::insert to return a bool.

  Change HandleSet::insert to return true if the handle was not already in the set.

  Change uses in naga::compact which were taking advantage of the prior return type of () to avoid needing to use semicolons in match arms.

• [naga] Use HandleSet in Validator::valid_expression_set.

  Add HandleSet methods new, clear, clear_for_arena, and remove.

  Change the type of naga::valid::Validator::valid_expression_set from BitSet to HandleSet. Adjust uses.

• [naga spv-out] Use HandleSet in ExpressionConstnessTracker.

  Change naga::back::spv::ExpressionConstnessTracker::inner from a BitSet to a HandleSet<Expression>. Adjust uses.

• [naga] Use HandleSet in Validator::needs_visit.

  Change the type of naga::valid::Validator::needs_visit from BitSet to HandleSet. Adjust uses.

  Add HandleSet method iter.

[ErichDonGubler]

This was also EZ to review. Kudos!

[jimblandy]

rebased, now that #5862 has been merged