dvec should implement iter::base_iter

[jesse99]

Note that it currently provides nothing in the way of iteration: you need to ignore the warning, use the internal data vector, do a null check, and then the iteration.

[nikomatsakis]

this is not true. it does implement iter::base_iter. make sure you import dvec::extensions

[jesse99]

I see: there is an extensions impl in iter-traits.rs that makes this work. Would be helpful if a doc comment in dvec.rs mentioned that the base_iter goo is supported.