Implement the <iterator> part of C++20 p1032 Misc constexpr bits.

[Smith-Rowland, Edward M]

Here is another chunk of p1032.
Jonthan,
I plan to get it all except maybe string::copy.  I can't figure out how to test without being able to create a constexpr string object. And anything I do will probably collide with your work on constexpr string. p1032 would be a tiny patch on top of that work anyway.
Ed

Test on x86_64-linux. OK?
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