* Proof of concept for "span" feature, with WGSL parsing augmented.

* Review:

1) add_span was actually a bad idea, make it set_span and add
set_span_if_unknown too.
2) panics on getting/setting span for invalid handles.
3) only set span for constants with a name
4) don't overwrite spans for types.

* Added spans to blocks & more expressions getting spans in frontends.

Definitely the shotgunny type of commit, but what can you do. The design
I went with made spans mandatory to specify, so I had to go and wire
them through wherever I could.

* Moved Block to a separate module, +clippy

* More spans for types in GLSL.

* Remove pointless body method.

* Make Arena interface require spans.

Another shotgun commit, oh boy...

* Fix tests.

My loathsome habit to "quickly fix things along the way" made a lot of
extra work for me here, having to fix my "fixes" for WGSL parser.

* Rustfmt + clippy.

* Fix compile-errors with span feature enabled.

* Nuked set_span* from orbit. Deleting code feels great!

* Code review - move feature flags inside functions.

* Fix build with "deserialize" feature enabled.