P2120 Simplified structured bindings protocol with pack aliases

[wg21bot]

P2120R0 Simplified structured bindings protocol with pack aliases (Barry Revzin)

[jfbastien]

Based on P1858 Generalized pack declaration and usage (#612).
Barry requests that I not schedule P2120 before P1858 is accepted.

[jfbastien]

Marking as needing revision, given the paper's dependencies.