Add Iterator::deref() for allowing use of smart pointers in iterators but use their inner value as a reference

[danakj]

Instead of

sus::move(it).map([](auto&& x) { return *x; }).map([](InnerT& t) { ... })

It would be nice

sus::move(it).deref().map([](InnerT& t) { ... })

Which does the same thing but more clear.

Smart pointers are super common in C++ so this is important, and the simple map function is very verbose compared to rust's .map(|x| *x).

This way code does not need to worry about what smart pointers are in use, they never appear in the signature either.

Currently some C++ smart pointers end up with implicit conversions to avoid the work of writing out their type and the dereferences.