[mypyc] Add vec iterators (#21135)

Add specialized `librt.vecs.vec` iterator classes. There are no
primitives for `iter(v)` yet, though we could add them later. Generally
using `iter(v)` should be avoided when using vecs, since the generic
iterator protocol is inefficient for unboxed items.

I used coding agent assist but reviewed generated code carefully (this
is mostly boilerplate).