The original pointer access test used SPIR-V for its input because WGSL didn't
have a working pointer indirection operator at the time. Now that it does, we
can just write this test in WGSL directly.

Fixes #1432.

Require the `accept` and `reject` arguments to `select` to be scalars or
vectors, per WGSL spec.

WGSL says:

> - The last member of the structure type defining the store type for a variable
>   ... may be a runtime-sized array.
>
> - A runtime-sized array must not be used as the store type or contained within
>   a store type in any other cases.

Thus, a struct whose final member is a struct whose final member is a
runtime-sized array is verboten.

Replace uses of `call_unique` with uses of `call` and `call_or`, which becomes
public. It's not clear when `call_unique` is correct to use, and avoiding a few
numeric suffixes here and there isn't worth it.

Eliminate `Namer::namespace_index` and remove namespace indices from
`Namer::unique` keys. Instead, implement `Namer::namespace` by just swapping in
a fresh table for the duration of the call.

Create the fresh hash table with the right initial capacity. The prior
implementation did manage to avoid allocations by sharing a hash table, and we'd
like to not lose that advantage entirely.

This has no effect on generated code.

* Remove unused petgraph dependency from glsl-out

* Emit std430 layout qualifiers for storage buffers

* Update WGSL grammar for pointer access.

Comes with a small test, which revealed a number of issues in the backends.

* Validate pointer arguments to functions to only have function/private/workgroup classes.

Comes with a small test. Also, "pointer-access.spv" test is temporarily disabled.

Automatically spills to a local variable function call arguments to
parameters expecting a pointer where the argument storage class isn't
function since the storage classes wouldn't match.

Treat expressions in `Function::named_expressions` like WGSL `let` declarations,
assuming that the Load Rule was applied to the rhs of the declaration, meaning
that their values are always `Indirection::Ordinary`.

Split `write_expr_plain_form` out from `write_expr_with_indirection`, to clean
up the parenthesis generation: no more `opened_paren` variable, just function
calls. This makes the early return for named expressions neater.

Fixes #1382.

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.