P2767 R2 flat_map/flat_set omnibus

[wg21bot]

P2767R0 flat_map/flat_set omnibus (Arthur O'Dwyer)

[JeffGarland]

@brycelelbach this needs LEWG attention. From the paper:

Some of the proposed solutions are LEWG-level design changes. Contrariwise, some of the issues collected here don’t have "solutions" at all, but are recorded merely For Your Information (for other vendors/implementors) to document the design choices libc++ has made.

Quick reading is that ‘at least these’ sections
4. Accidentally explicit constructor
5. Add move semantics to flat_set::insert_range
6. Add move semantics to flat_map::insert_range
7. insert is more primitive than emplace

[wg21bot]

P2767R1 flat_map/flat_set omnibus (Arthur O'Dwyer)

[wg21bot]

P2767R2 flat_map/flat_set omnibus (Arthur O'Dwyer)