<deque>: shrink_to_fit() should follow the Standard

[StephanTLavavej]

WG21-N4958 [deque.capacity]/6:

Effects: shrink_to_fit is a non-binding request to reduce memory use but does not change the size of the sequence.
[Note 1: The request is non-binding to allow latitude for implementation-specific optimizations. - end note]
If the size is equal to the old capacity, or if an exception is thrown other than by the move constructor of a non-Cpp17CopyInsertable T, then there are no effects."

We unconditionally move, which is a bug:

STL/stl/inc/deque

Line 998 in 283cf32

deque _Tmp(_STD make_move_iterator(begin()), _STD make_move_iterator(end()));

(#4071 is patching this line in an unrelated way.)

vector provides its strong guarantees correctly, like this (note that it doesn't directly use move_if_noexcept()):

STL/stl/inc/vector

Lines 837 to 842 in 283cf32

	if (_Whereptr == _Mylast) { // at back, provide strong guarantee
	if constexpr (is_nothrow_move_constructible_v<_Ty> || !is_copy_constructible_v<_Ty>) {
	_Uninitialized_move(_Myfirst, _Mylast, _Newvec, _Al);
	} else {
	_Uninitialized_copy(_Myfirst, _Mylast, _Newvec, _Al);
	}

[frederick-vs-ja]

Reallocation of elements doesn't seem valuable since the current block size of deque is unwisely small. I think we should just deallocate unused blocks and shrink the internal map.