US 48-108 26 [ranges] Add views::enumerate

[wg21bot]

views::enumerate is one of the most commonly used adaptors and it would be embarrassing if we don’t ship it because we spent so much time making it possible to give it named members.

Proposed change:

Adopt P2164

[brycelelbach]

The proposed resolution is to adopt P2164 cplusplus/papers#875

[jensmaurer]

Removing the SG9 label.