std: Move the iterator param on FromIterator and Extendable to the metho...

...d.

If they are on the trait then it is extremely annoying to use them as
generic parameters to a function, e.g. with the iterator param on the trait
itself, if one was to pass an Extendable<int> to a function that filled it
either from a Range or a Map<VecIterator>, one needs to write something
like:

    fn foo<E: Extendable<int, Range<int>> +
              Extendable<int, Map<&'self int, int, VecIterator<int>>>
          (e: &mut E, ...) { ... }

since using a generic, i.e. `foo<E: Extendable<int, I>, I: Iterator<int>>`
means that `foo` takes 2 type parameters, and the caller has to specify them
(which doesn't work anyway, as they'll mismatch with the iterators used in
`foo` itself).

This patch changes it to:

    fn foo<E: Extendable<int>>(e: &mut E, ...) { ... }