Treat `TypeInner::ValuePointer` and `TypeInner::Pointer` as equivalent by
converting them to a canonical form before comparison.

Support `ValuePointer` in WGSL type output.

Fixes #1318.

`UniqueArena::fetch_or_append` becomes `insert`.

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

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.

Fixes #1385.

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.

Previously the typifier flagged all relational functions as producing a
scalar boolean but with the exception of `all` and `any`, according to
the wgsl spec, all other relational functions output a type the same
size as the input.

Adds checks that it isn't used in an unsupported function and emits the
depth_ref as a separate argument.

Add support for float, vector and matrices targets.

Fix prefix and postfix being inverted (one was returning the value
of the other).

Remove an unneeded local indirection for prefix handling.

Add tests.

This caused arrayed textures to not be accepted

Constants that are to be dynamically indexed now are first transformed
into locals with them as the initializer.

Fixes #1352.

Without this change, the `to_range` method method returns `None` even when spans
are enabled.

Fixes #1356.

Output for the test case in that issue is now:

    error: the value indexed by a `[]` subscripting expression must not be a pointer
      ┌─ wgsl:5:14
      │
    5 │     let a = *pv[3]; // Problematic line
      │              ^^ expression is a pointer

    Could not parse WGSL