P0009: Fix mdspan.subspan definition

@crtrott @dsunder

1. The code defining first, last, and R is not valid source code, because it uses a run-time value r to get the r-th entry of a parameter pack slices....
2. R needs to be constexpr, because Ensures needs the R[k]-th entry of a parameter pack slices....
3. We need a different notation for R, since we use that elsewhere in the document as the rank (an integer value, not an array).

I think we could define R with math instead of code. We can say in a Note that we know its length and entries at compile time. This would make it easier to define first and last in code.

Locate the mdspan.subspan definition in the proposal documents and read the surrounding definitions of first, last, R, and rank. Check the existing notation and the Ensures wording; the work is done when the parameter-pack indexing is described with valid compile-time notation and R is distinct from the document's rank value.