* 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.

This generator collects values from the underlying generator until it
has a specified amount of them, and then returns them in one "chunk".
In case the underlying generator does not have enough elements for
a specific chunk, the left-over elements are discarded.

Closes #1538

This means mutiple generic generators and some inference helper
* take(n, generator)
* filter(predicate, generator)
* map(func, generator)
* repeat(generator, repeats)