Move the parsing of type construction expressions into a function of its own,
which can return `None` when the identifier at hand is not the name of a type.
This means that `parse_primary_expression` doesn't have to commit to
interpreting the expression as a type construction too early, allowing it to
report unknown identifiers as Error::UnknownIdent.

[wgsl-out] Implement all math functions. Rename entry point function based on stage. Rename entry point output struct.

This lets us reduce noise by removing a bunch of `#[cfg]` attributes. In the
future, if we have tests for other front ends, we can create an `errors`
subdirectory, but for now, we only have the WGSL tests, so we should keep things
flat.