Make str::from_utf8 const fn

[lf-]

However: I am not at all confident in the soundness of this
implementation. I am getting around the lack of const align_offset by
writing the alignment myself by casting to usize. However, this is marked
unsafe. After searching the rustc source for why, I found this:

// casting pointers to ints is unsafe in const fn because the const evaluator cannot
// possibly know what the result of various operations like `address / 2` would be
// pointers during const evaluation have no integral address, only an abstract one

And I don't know if an "abstract" address can have binary & applied to it, so I'd like some help with that.

[rust-highfive]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @dtolnay (or someone else) soon.

If any changes to this PR are deemed necessary, please add them as extra commits. This ensures that the reviewer can see what has changed since they last reviewed the code. Due to the way GitHub handles out-of-date commits, this should also make it reasonably obvious what issues have or haven't been addressed. Large or tricky changes may require several passes of review and changes.

Please see the contribution instructions for more information.

[leonardo-m]

Perhaps it's better to make align_offset const first (or at the same time).

[lf-]

I need to go understand the const evaluator a lot more I think, because the spooky comment is supposedly right after I talked to someone more familiar with that side: there are not integral pointers in the constant evaluator. So this feature might require some kind of compiler support or making it so that align_offset returns usize::MAX for all inputs in constant evaluation mode. I'm not entirely sure what to do given this.

[lf-]

It looks like I've wandered straight into a proverbial patch of brambles: #62420 (comment)