The static member variable version was flawed as it suffered from SIOF,
since TextPos' static END made use of Index's static LAST. Often, END
would be initialized before LAST resulting in END being invalid/garbage.

This approach, while slightly less terse, prevents that issue.

The only other option is to use static member functions that return a
type constructed to be that extent, which would enact RAII.

The interface would look like: Index32::last();
instead of Index32(Index32::Last).

This also fixes ambiguity warnings introduced with C++20 due to the old
operators not having proper const correctness.

Needs to be explicit so they can be used before they are defined in
other translation units.