inconsistent indenting for multi-lined requires clauses

[burblebee]

We're inconsistent when indenting requires clauses which span multiple lines.
Most indent subsequent lines to match up with what follows the requires keyword;
some indent 2 spaces (under the 'q' of the requires keyword).

[JohelEGP]

The motions add many of those within parentheses, at least one even preceded by !. The decision needs to include wether these should be further indented after the parentheses:

requires (forward_iterator<I> || random_access_iterator<O>) &&
         indirectly_copyable<I, O> &&

And a generalization of that rule:

    concept @\defexposconcept{convertible-to-non-slicing}@ =                    // \expos
      convertible_to<From, To> &&
      !(is_pointer_v<decay_t<From>> &&
      is_pointer_v<decay_t<To>> &&

[jensmaurer]

Editorial meeting: <algorithm> indents to after the "requires" token. Tentatively agreed.