layout_stride mapping constructor taking (extents_type, span) doesn't compile

[mhoemmen]

@youyu3 discovered that the layout_stride::mapping(const extents_type&, span<OtherIndexType, rank()>) constructor doesn't compile. Here is an example:

https://godbolt.org/z/Tf57xdrGq

    using extents_type = stdex::dextents<int, 2>;
    extents_type ex{2, 2};

    // this one works fine
    stdex::layout_stride::mapping map0{ex, std::array{3, 3}};

    std::array<int, 2> strides{3, 3};
    std::span<int, 2> strides_span{strides.begin(), strides.end()};

    // Neither of these compile 
    stdex::layout_stride::mapping map1{ex, strides_span};
    stdex::layout_stride::mapping<extents_type> map2{ex, strides_span};

@youyu3 suggests a fix: add a __deduction_workaround::fill_strides overload taking span<IntegerType, extents_type::rank()>, like the existing one that takes array<IntegerType, extents_type::rank()> ( https://github.com/kokkos/mdspan/blob/760060059fb746018a9849234e02dc9bf003861b/include/experimental/__p0009_bits/layout_stride.hpp#L197 ):

template<class IntegralType>
MDSPAN_INLINE_FUNCTION
static constexpr const __strides_storage_t
fill_strides(const span<IntegralType, extents_type::rank()>& s) {
  return __strides_storage_t{static_cast<index_type>(s[Idxs])...};
}