Non-exhaustive matches on `Expression` make it harder to review changes that add
new enum variants, because the compiler can't tell you that you've missed a
case.

Although I believe Clippy is able to do so, I don't think we should make a
blanket rule of forbidding wildcard match arms against `Expression`. There are a
few matches that are only concerned with a handful of variants, or a single
variant. For example, variant, naga::front::spv::Parser::next_block is only
concerned with whether an expression is a constant or not. In principle, these
sites should also be checked when adding a new variant, but it seems too
burdensome to require them to list all the `Expression` variants they don't care
about. The principle should be applied only to matches that are already handling
most variants.

At the moment, there is only one such match on `Expression` in the code base,
and it's only missing a few variants, so this is quick to fix.

The snapshot files in tests/out start with headers like this:

    ---
    source: tests/snapshots.rs
    expression: dis
    ---
    ; SPIR-V
    ; Version: 1.0
    ; Generator: rspirv
    ...

The Makefile's `validate-foo` targets trim off those lines with `tail -n +5`
before submitting their test to the validators.

However, some versions of the `insta` crate seem to generate headers with an
extra blank line, like this:

    ---
    source: tests/snapshots.rs
    expression: dis

    ---

This makes the `validate-foo` targets unhappy.

This commit changes the `validate-foo` targets to use `sed` to cope with both
kinds of headers.

* Make constant composites non-constexpr

* Update shadow test

* Add a note about references and pointers to Expression::Load

* Formatting

* Remove comma

* Clearer English

* Handle length(float x)

* Handle length(float) and distance(float, float)

* Apply suggestions

* Apply suggestions

* Move stack size range up by 1000

* Take references in function calls if thats what the funciton wants

* Use references instead of pointers the function argument list

* Add hlsl-out feature

* Integrate hlsl-out with snapshots

* Add snapshot

* Handle NaN and inf

* Handle negative infinity

beautify

beautify #2

This adds `use` declarations for `spirv::Decoration` and `spirv::BuiltIn` local
to functions where those are used repeatedly.

The initial intention was to make the code a little more elegant and match on
the Vec itself instead of its length, however there's also a - probably
minuscule - performance advantage:

`Vec::remove` internally copies the removed element onto the stack. In this case
that's a `StructMember` weighting 48 bytes. The `Binding` we are actually
interested in is only 8 bytes tough.

Cloning just the `Binding` saves us from copying 40 bytes for nothing.

Also move out "allow_point_size" into SubOptions to allow
main Options to not be cloned.