Override-expressions for Fixed Size Arrays

[kentslaney]

This pull request is still a work in progress, but is open to avoid others repeating work that's already been done.

Connections
Resolves #5315

Description
The WebGPU spec allows for arrays to have a fixed size specified by an override-expression. This PR implements this by adding a Pending state to array sizes, which gets resolved with the rest of the override-expressions.

Testing
This change is tested by checking that a 1d spiral is correctly stored.

Checklist

• Run cargo fmt.
• Run taplo format.
• Run cargo clippy. If applicable, add:
• Run cargo xtask test to run tests.
• Add change to CHANGELOG.md. See simple instructions inside file.

[teoxoy]

We need to validate that arrays of this form are only declared on workgroup variables. They also can't be nested. I think this can be done via a new TypeFlags implementing the spec's distinction between "creation-fixed footprint" and "fixed footprint".

see https://www.w3.org/TR/WGSL/#fixed-footprint-types & https://www.w3.org/TR/WGSL/#example-workgroup-variables-sized-by-override

[teoxoy]

There is also this note in the spec:

Note: To qualify for type-equivalency, any override expression that is not a const expression must be an identifier. See Workgroup variables sized by overridable constants

I think to make this work we'd have to make ArraySize::Pending hold a new enum that is either a Handle<Override> or Handle<Expression>.

[kentslaney]

I think to make this work we'd have to make ArraySize::Pending hold a new enum that is either a Handle<Override> or Handle<Expression>.

You were right, that was much easier, sorry for the force push

[teoxoy]

Looks great, thanks!