P1684: Permit array size to be larger than needed

[mhoemmen]

This comes from 1684R2 LEWG review on 2022/04/19.

Currently, P1684 requires array to have size() equal to required_span_size(). Relax this to let array's size() be greater than or equal to required_span_size(). This would permit e.g., SIMD-aligned access.

[crtrott]

Ha I had it like that, but some specific folks complained :-)