flat_map should support same interface as std::map

[J-Richter]

std::map has a transparent erase function. flat_map should support the same interface.

This means flat_map can be used with fewer allocations like so:

boost::container::flat_map<std::string,int,std::less<void>> v;
v.erase( "test"sv );

flat_map should also provide the insert_or_assign function.