[Rigidity]

The type checker's comparison, intersection, substitution and memoization keep their maps and sets keyed by small integer IDs, and hashing those with SipHash was a large part of their run time. This switches them to FxHash (rustc-hash 2).

Changes

• Added rue-types/src/hash.rs, which re-exports FxHashMap and FxHashSet and defines the FxIndexMap and FxIndexSet aliases.
• memo.rs, compare.rs (the ComparisonContext stack, bindings, references, rigid and monotone sets), intersect.rs, substitute.rs (the stack, memo and renames) and check.rs (the check memo and the overlap stack) use them.
• The HashMap<TypeId, TypeId> taken by the public substitute_with_mappings is left as it is.
• Nothing in the public API changes.

Timings

These are debug builds, compared against 65b2f24d:

Test	Before	After
test_memo_differential	18.7s	13.2s
test_check_property	7.4s	6.2s
test_check_property_generics	7.7s	6.4s

Together with #294, the CPU time of the rue-tests snapshot suite went from 37.2s to 24.8s.

The ignored test_memo_differential_large still passes, taking about 5.5 min in release.