RU371 [lib] Apply "Mandating the Standard Library" papers

[wg21bot]

Apply all the wordings from all the “Mandating the Standard Library” papers.

Proposed change:
Apply wordings from P1505, P1622, P1686, P1718-P1723.

[JeffGarland]

Note P1505 is superseded by P1723. As of Thu in Belfast 1622, 1686, 1720, 1721, 1722 have been reviewed and adopted.

[JeffGarland]

As of 2019-11-07 all these papers are ready to vote for c++20 in Belfast.

[jensmaurer]

LWG in Belfast: Accepted.