Ranges (C++26)

Adds views::concat.

Removes the common-reference requirement from the indirectly invocable concepts.

Retires niebloids: the range algorithms are specified as function objects.

Adds views::cache_latest, caching the current element so an expensive transform runs once per element.

Adds ranges::reserve_hint and approximately_sized_range so lazy ranges can hint their size to ranges::to.

Adds views::to_input, which downgrades a range to an input range to avoid costly multi-pass guarantees.

Adds views::indices(n), an index range from 0 to n.

Makes the constructors of view iterators and sentinels private/exposition-only (DR).

Extends filter_view to make the classic misuse (modifying elements so they stop satisfying the predicate) safer (DR).

Renames views::to_input to views::as_input.

Adds bounds-checked at() to view_interface.