Ensure that each distinct type occurs only once in Module::types

[jimblandy]

Each commit should be CI-safe. I recommend reviewing commits separately.

• Use UniqueArena for types.

  Ensure that each distinct type occurs only once in Module::types, so that we
  can use Eq on Type or TypeInner for type equivalence, without being
  confused by differing Handle<Type> values that point to identical types.

  This removes a number of duplicate types from the ir snapshots.

• Simplify interpolation defaulting.

  Replace Module::apply_common_default_interpolation with a simpler function
  that handles a single Binding at a time. In exchange for the simplicity, the
  function must be called at each point function arguments, function results, and
  struct members are prepared. (Any missed spots will be caught by the verifier.)

  This approach no longer requires mutating types in the arena, a prerequisite for
  properly handling type identity.

  Applying defaults to struct members when the struct declaration is parsed does
  have a disadvantage, compared to the old whole-module pass: at struct parse
  time, we don't yet know which pipeline stages the struct will be used in. The
  best we can do is apply defaults to anything with a Location binding. This
  causes needless qualifiers to appear in some output. However, it seems that our
  back end languages all tolerate such qualifiers.

• Consolidate Handle construction code.

• [glsl-in]: Register new types as necessary during constant solving.

• Rename UniqueArena methods to more closely resemble HashSet.

  UniqueArena::fetch_or_append becomes insert.

  UniqueArena::try_get becomes get_handle, by analogy with get, that takes a
  type as a key.

[kvark]

That's wonderful, I was looking forward to this change! And well executed, too 👍 🎖️ . Just one note to address.

[jimblandy]

That last forced push just adds a "fixes #1385" note to the commit message. No code change.