[naga wgsl-in] vecN() constructors and let type conversions

[andyleiserson]

Connections
Fix for #7356.

This pull request is on top of #7304, due to overlapping effects on naga output snapshots.

Description
Naga did not previously allow use of vec4() as a constructor, because it has no type information. However, the spec states that this constructor (and vec2/vec3) should return a vector of abstract ints. Depending on the context (e.g. the case of var v: vec4f = vec() from the associated issue), the vector of abstract ints may then be converted to another type. While writing test cases for this, I noticed that we weren't applying automatic conversions to the initializer of let declarations, so I fixed that as well, changing the code for handling let declarations to operate similarly to the code for var and const, which already handled conversions.

Testing
Added several snapshot tests to exercise various related cases, plus a few cases where a conversion is not possible in wgsl_errors. Type conversion for let declarations also fixes the CTS test webgpu:shader,validation,decl,let:type:case="array_sized".

Squash or Rebase? Squash

Checklist

• Run cargo fmt.
• Run taplo format.
• Run cargo clippy --tests. If applicable, add:
  • --target wasm32-unknown-unknown
• Run cargo xtask test to run tests.
• Needs a CHANGELOG.md entry.

[jamienicol]

Love the links to the spec and the thorough test coverage 🥇

[jamienicol]

Great, LGTM!

[ErichDonGubler]

Rebased to resolve conflicts.

[ErichDonGubler]

@andyleiserson, @sagudev: Should we just merge this, and close #7304? Or do we need to merge that first?

[andyleiserson]

I don't know if @jamienicol reviewed all the changes or just the ones that I added in this PR. And @jimblandy mentioned documenting ImplConst, which made me question whether there is some issue removing it.

[jamienicol]

I only reviewed your changes @andyleiserson