Add a new Naga feature, `"compact"`, which adds a new function
`naga::compact::compact`, which removes unused expressions, types, and
constants from a `Module`.
* build: move to the Rust 2021 edition
Since the MSRV of `naga` [is currently 1.56][msrv], I don't think there's a strong reason to stay
with the 2018 edition, and there _are_ a [few good reasons][edition-guide] to move to the 2021
edition.
I did this migration mostly automatically, per [official Rust guidelines]:
```sh
$ cargo fix --edition --all-targets
$ sed -i Cargo.toml 's/2018/2021'
$ cargo fix --edition-idioms --allow-dirty # doesn't change anything
```
The only manual edit needed to stymie a new warning introduced was the removal of the `TryFrom`
import in several modules, since it's now in the 2021 prelude.
[msrv]: a7193d652e/.github/workflows/pipeline.yml (L14)
[edition-guide]: https://doc.rust-lang.org/edition-guide/rust-2021/index.html
[official Rust guidelines]: https://doc.rust-lang.org/cargo/commands/cargo-fix.html#edition-migration
* refactor(wgsl-in): use `pat` instead of `pat_param` again
How we were using `pat` in the Rust 2018 edition is actually the use case that
Rust 2021's `pat` fragment specifier is intended to satisfy. So, let's just use
that!
When it was introduced it was supposed to allow for fast compiles by
skipping glsl specific validation, but as it turns the subset of glsl that's
compilable code is already pretty close to the subset of valid glsl code.
So the current code gated behind glsl-validate amounts to a single branch that
isn't even performance sensitive, and most of the validation is not specific to
glsl and is made by naga's validator which can be turned off, so the original
goal of fast compile times by disabling validation can still be accomplished.
