Eliminate redundant into_iter() functions in favor of IntoIterator

[theemathas]

Currently, many collections (including Vec) both implement the IntoIterator trait (which has .into_iter() as a required method) and implement .into_iter() as an inherent method. I think that we should remove the inherent method and put the IntoIterator trait in the prelude.

[aturon]

cc me, this was an oversight that can likely be rectified before 1.0

[aturon]

See #1030