Allow span(std::array<const T,N>) ?

[martinmoene]

span-lite release 0.1.0 allows the construction of a span from std::array with a const value type.

However this is not supported by p0122r7.

Keep an eye on this and if needed, decide to allow or not such a difference from std::span. If it's different from std::span, add a notice to the test specification.

See also thread [isocpp-lib] P0122R7 (span): element_type instead of value_type in constructor taking std::aray? of 28-Apr-18, 15:21.