WGSL requires that runtime-sized arrays appear only as the last member of a
structure in in the `storage` storage class. It seems to me that Naga should
enforce this restriction on its own IR as well.

The `apply_common_default_interpolation` helper function would panic if bindings
were missing, but missing bindings should be something that front ends can count
on validation to detect, so the helper should just return silently.

The validator returned `InvalidType` errors for missing bindings, apparently
because variables without bindings must be structs that do have bindings. But
this is unhelpful when you've just forgotten to label an argument. So this patch
adds a new, more specific, `VaryingError` variant.

The expressions on both sides must now have the same dimensions
otherwise a semantic error is returned.

Previously we passed a struct with the input bindings and just ignored
the outputs. Now we handle them the same way we do with builtins, by
creating function local globals that are read and written in a wrapper
entry point.

Rename TokenMetadata to SourceMetadata to reflect it's actual use

Co-authored-by: João Capucho <jcapucho7@gmail.com>