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.

Renames:

Error::UnknownDecoration => Error::UnknownAttribute
TypeDecoration => TypeAttributes
Scope::Decoration => Scope::Attribute

various argument names, string literals, comments

* Panic to prevent arena Handle overflow

* Use `NonZeroU32::new` to detect overflow