P0009: Add compile-time slices to submdspan

submdspan currently takes three types of slice parameters ("SliceSpecifiers").

1. full_exent_t (analogous to Fortran or Matlab colon)
2. A type convertible to index_type (size_t in P0009R16, the last P0009 revision to include submdspan)
3. A type convertible to tuple<index_type, index_type> (tuple<size_t, size_t> in R16)

The return type depends only on the types of the input, not their values. Thus, even though submdspan is constexpr, it's impossible to use the value of an index_type or tuple parameter to determine the type of the layout. This matters for custom layouts that represent all extents and strides information as compile-time constants.

One way to fix this would be to allow two more categories of slice parameter types.

4. A type convertible to integral_constant<index_type, k> for some index_type value k
5. A type convertible to tuple<integral_constant<index_type, j>, integral_constant<index_type, k>> for some index_type values j and k

These categories would also permit optimization for existing layouts, by letting the returned mdspan have compile-time extents if possible.

Start by reading the existing submdspan SliceSpecifiers description and P0009R16, especially how input types determine the returned layout. Develop a proposal for the two compile-time slice categories and specify their effect on returned mdspan types, including compatibility with existing layouts. Done means the semantics and type consequences are clearly documented for committee review.