Optimize vec::MoveIterator for POD types

[erickt]

vec::MoveIterator is written in such a way to be safe for dtors, but this is unnecessary overhead for POD types. Because vec is such an important type, it's worth optimizing for this case. I think all we need is to make use of std::libstd::needs_drop(), but I'm not completely sure if that's all that is needed.

[erickt]

I have an experimental implementation of this in this gist. It's roughly twice as fast, but I'm not sure if it's safe yet. It's still 10% slower than v.iter().map(|x| *x) though.