[wgsl-in] support const-expressions in attributes

allow `Splat` as an evaluated const-expression type

With the addition of the constant evaluator to the `proc` module, it
is now concerned with constructing expressions. Any code that
constructs expressions will generally also need to deal with `Emit`
statements, which are handled by the `Emitter` type. However,
`Emitter` is private to the `front` module. This patch moves it to
`proc` and makes it accessible to both the constant evaluator and the
front ends.

Instead of letting the user supply an arbitrary closure for appending
expressions, instead give `ConstantEvaluator` an `Option` that holds
an `Emitter` to interrupt, and a `block` to add any new `Emit`
statements to.