Some changes occurred to the CTFE machinery

cc @RalfJung, @oli-obk, @lcnr

callers of file_with_nul will make an unnecessary call to strlen when they just need to pass the pointer into C code.

I don't think that this is really a disadvantage, as the compiler is perfectly able to remove an unnecessary call to strlen (eg see https://godbolt.org/z/3ezaz1esa).

There is still a plan to make &CStr a thin pointer that does not know the string length. It's just blocked on figuring out how to make !Sized types more flexible.

We discussed this in the libs-api meeting. There wasn't much enthusiasm for this change.

Part of the reason is that we might still make &CStr a thin pointer. Even if we don't, using a raw pointer here is a very inconvenient type. Reducing the size of Location by not storing the length doesn't seem to have much advantages, and does mean having to use strlen for the regular file() -> &str method. And even if we do end up going that route (while keeping &CStr wide), the strlen call can be easily optimized away as pointed out above.

Because of these reasons, I'm closing this PR.

Thanks! Marking the feature as implementation complete in the tracking issue.