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.
