<mdspan>: Cache fwd-prod-of-extents and rev-prod-of-extents when all extents are static

[JMazurkiewicz]

When all extents are static we can cache fwd-prod-of-extents and rev-prod-of-extents:

• _Fwd_prod_of_extents and _Rev_prod_of_extents are not member functions of extents anymore (they are classes),
• When all extents are static, then those classes contain static array with all possible fwd or rev products.

After getting WG21-P2647R1 in MSVC we can restore member functions and make caching arrays local to them.

[StephanTLavavej]

Thanks for this neat perf improvement! 🚀 🐇 🐆