Make naga::back items public to support out of tree backends

[LegNeato]

Is your feature request related to a problem? Please describe.

As suggested in a matrix thread, I am trying to convert the rust backend in #5175 to an out of tree maintained backend. To do so, I need some types in naga::back to be public.

For example, my PR uses

wgpu/naga/src/back/mod.rs

Line 77 in a63bec8

struct FunctionCtx<'a> {

which works when it is in-tree but fails out of tree.

It seems pretty much everything in naga::back should be public if out of tree backends are encouraged as the module is meant for backends to use.

I'm fine dealing with breakage if these are pub and marked as unstable and/or hidden in the docs.

Describe the solution you'd like
The items in naga::back be pub.

Describe alternatives you've considered

1. I could not use those abstractions. My out of tree backend will be structured differently / drift from the in-tree ones. Eventually I would like to merge it in naga proper and using existing abstractions will help.
2. I could copy and paste what I need in. This will be what I do if naga doesn't want them to be pub.

Additional context
I was told in matrix to open an issue when something prevents the backend from being out of tree. I am happy to put up a PR flipping what I need to pub, but I wanted to open this issue first to see if there is a different preferred solution.

[AdrianEddy]

+1 I'm also interested in this feature