This changes all of the STL's operators that are required to exist, but that aren't required to be namespace-scope functions, into "hidden friends". (For example, n + vector::iterator.)

Hidden friends are great in two different ways:

• They improve compiler throughput, because they don't pollute unqualified name lookup. Only argument-dependent lookup finds them when they're needed.
• They automatically work with named modules, when their parent class (or a function returning their parent class, etc.) is exported. (Namespace-scope operators would have to be individually exported.)

A few notes:

• span::iterator has used hidden friends since <span>: fix cross-type iterator operations #474 and currently says:

  STL/stl/inc/span

  Line 117 in ea32e86

  _NODISCARD_FRIEND constexpr _Span_iterator operator+(const difference_type _Off, _Span_iterator _Next) noexcept {

• The <iomanip> diff looks horrible, but I've structured it into a series of commits for easier reviewing.
  • The _Elem and _Elem2 template parameters have to "trade places" when becoming hidden friends. We static_assert that they're the same, so there is no potential for mistakes.
• quoted() needed extra attention due to how its operators referred to each other. I recommend diffing the "before" and "after" parts of each commit.
  • First, I made this easier to deal with by moving _Quote_out before _Quote_in.
  • In "Use friendship for _Quote_out.", I had to rename to _OsTraits to avoid a collision, and use class _QuTraits = _Traits both for clarity and to delay evaluation of is_void_v<_QuTraits>.
  • In "Use friendship for _Quote_in, part 1.", instead of calling quoted(), I directly construct a _Quote_out (thanks to the earlier code movement).
  • Finally, "Use friendship for _Quote_in, part 2." is nearly pure code movement.

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