P3471R4 Standard library hardening

[Eisenwave]

Fixes #7674.
Fixes cplusplus/papers#2125.

[Eisenwave]

@jwakely I have had some issues when creating this PR:

• The new note for [string.view.access] looks as if it had a paragraph number, but it's most likely intended to be attached to the hardened precondition paragraph. I have elected to ignore the paragraph number.

• The paper deletes "Two kinds of implementations are defined" in [intro.compliance] but the library has the same sort of wording in [compliance] (library intro). How to proceed? Delete redundant wording? Mirror changes?

• [structure.specifications] in the paper explains "Hardened preconditions" simply with two bullets that begin with "When". This is not how we usually do things. My suggestion would be something like "If the implementation is hardened ([intro.compliance]), ..." for the first bullet, and "Otherwise, ..." for the second bullet.

• [basic.contract.eval] is a dead reference at this point. It should be resolved after merging with contracts.

• It was very annoying that the paper doesn't have a wording diff that follows the structure of the standard linearly. For example, the paper first has the wording diff for 23.7.3.6.3 [mdspan.mdspan.members], and then the diff for 27.7.3.6.2 [mdspan.mdspan.cons]. The only thing this accomplishes is to bully the editor, and I'm the editor, and I feel bullied!