P1004 Making std::vector constexpr

[jensmaurer]

P1004R1 Making std::vector constexpr (Louis Dionne)

https://issues.isocpp.org/show_bug.cgi?id=481

[mclow]

This was discussed in San Diego, but is waiting on Core to pass P0784 More constexpr containers #110.

[wg21bot]

P1004R2 Making std::vector constexpr (Louis Dionne)

[wg21bot]

Adopted 2019-07.