auto merge of #8604 : kballard/rust/iter-size-hint, r=graydon

Implement `size_hint()` on the new std::vec Iterators.

Add or update `size_hint()` on std::iterator Iterators where appropriate.

r? @thestinger

diff --cc src/libstd/iterator.rs
Simple merge

diff --cc src/libstd/vec.rs
Simple merge