[New pipes] pipes::values and pipes::keys for std::map ; pipes::erase

[Guekka]

Hey,
In the range-v3 library, there are ranges::values and ranges::keys that can be used to only send the keys or the values of a map.
We can use pipes::unzip for that, but it is kinda ugly to write
map >>= pipes::unzip(pipes::dev_null(), /*Do something*/);
So I think pipes::value and pipes::keys would be a useful addition to this library

Edit : Just got a new idea : pipes::erase
It could be used to replace std::erase like that
cont >>= pipes::erase(predicate);