Introduce a new struct `Scalar`, holding a scalar kind and width, and
use it as appropriate in the WGSL front end. This consolidates
many (kind, width) pairs, and lets us name the two components.
Ideally, `Scalar` would be used throughout Naga, but this would be a large
change, touching hundreds of use sites. This patch begins by
introducing `Scalar` to the WGSL front end only.
This appears to match other backends, and fixes
fix the case where expressions which were named in earlier
functions are used in local variable declarations
Replace the `TypedExpression` struct, used to distinguish between WGSL
pointers and references since Naga has only `Pointer`, with an enum,
`Typed`, with variants for references and plain types. This cleans up
a bunch of code, since the struct's `is_reference` field basically
served as a detached enum discriminant. This also prepares the code
for adding abstract types.
When an error snapshot test fails and we generate a diff comparing the
expected output with the actual output, treat the expected output as
the diff "from", and the actual output as the diff "to" - not the
reverse.
