* Deduce map return type implicitly
Giving the first template argument to map generator function to deduce
return type is now optional even if the return type is different from
the type generated by mapped generator.
The support is to be considered experimental, that is, the interfaces,
the first party generators and helper functions can change or be removed
at any point in time.
Related to #850
There are still some holes, e.g. we leave surrogate pairs be
even though they are not a part of valid UTF-8, but this might
be for the better -- WTF-8 does support surrogate pairs inside
text.
Closes#1207
This allows reducing the amount of friends needed for its interface
and some extra tricks later.
The bad part is that the pointer can become invalidated via
calls to other StringRef's public methods, but c'est la vie.
