[container.adaptors] Reorder constructors for flat_* adaptors (#6274)

Canonicalize the ordering of the constructors for the flat_* adaptors:
 - default constructor is first
 - each overload without `key_compare` is followed by the corresponding one with `key_compare`
 - each pair of overloads without `sorted_unique` is followed by the corresponding pair with `sorted_unique`

Author: Quuxplusone