[naga] Remove uses of Handle::index for identifier generation from backends

[jimblandy]

Remove all uses of Handle::index for generating identifiers from Naga backends.

This is a step towards making Handle::index less prominent in the code base. Eventually, I'd like to use well-typed collections for everything, and have Handle::index be private to arena.rs.

• [naga msl-out] Use Handle::write_prefixed instead of index.

  This replaces all uses of Handle::index for generating identifiers
  in naga::back::msl with uses of Handle::write_prefixed.

  There are still some uses of Handle::index remaining.

• [naga dot-out] Use Handle::write_prefixed instead of index.

  This replaces all uses of Handle::index in naga::back::dot with
  uses of Handle::write_prefixed.

• [naga hlsl-out] Use Baked for baked expressions.

  Make the HLSL backend more like other backends by using back::Baked
  to generate names for baked expression identifiers. This removes the
  final uses of Handle::index from the HLSL backend.

  This is separated out from the previous commit because it changes lots
  of snapshot tests, whereas the previous commit has no effect on Naga's
  output.

• [naga] Introduce Baked newtype for writing baked expression names.

  Introduce a newtype naga::back::Baked that wraps a
  Handle<Expression> and formats with std::fmt::Display as a baked
  expression identifier. Use this in all backends for generating baked
  identifiers.

  Delete BAKE_PREFIX, as it's no longer used outside of Baked's
  Display implementation.

[teoxoy]

Nice!