Currently, the key-extracting optimization mechanism incorrectly catches some volatile-qualified glvalues (due to _Remove_cvref_t) and attempts to bind them to a const K& reference, which makes the program ill-formed. This PR make the mechanism never catch volatile glvalues, which makes volatile accepted under some conditions as specified by the standard.

In piecewise construction, when the first tuple argument is a 1-tuple, it's sometimes possible to directly treat its only tuple element as the key. Also, since C++23, it should be possible to directly get the key from suitable tuple<K, V> and array<K, 2> without additional construction. This PR attempts to extend the optimization mechanism to catch these cases.

The mechanism should also be usable for flat_set and flat_map.

Note that subrange is intentionally ignored because the corresponding get function copies (or moves) the key, which doesn't seem desired.