`<random>`: Enforce `seed_seq::generate()`'s mandate for `RandomAccessIterator`

[StephanTLavavej]

WG21-N4971 [rand.util.seedseq]/7:

template<class RandomAccessIterator>
  void generate(RandomAccessIterator begin, RandomAccessIterator end);

Mandates: iterator_traits<RandomAccessIterator>::value_type is an unsigned integer type capable of accommodating 32-bit quantities.

We currently don't enforce this:

STL/stl/inc/random

Lines 180 to 184 in 8b081e2

	template <class _RanIt>
	void generate(_RanIt _First, _RanIt _Last) const { // generate randomized interval from seeds
	_Adl_verify_range(_First, _Last);
	auto _UFirst = _Get_unwrapped(_First);
	const auto _Nx = static_cast<size_t>(_Get_unwrapped(_Last) - _UFirst);