Implement Default for some alloc/core iterators Add `Default` impls to the following collection iterators: * slice::{Iter, IterMut} * binary_heap::IntoIter * btree::map::{Iter, IterMut, Keys, Values, Range, IntoIter, IntoKeys, IntoValues} * btree::set::{Iter, IntoIter, Range} * linked_list::IntoIter * vec::IntoIter and these adapters: * adapters::{Chain, Cloned, Copied, Rev, Enumerate, Flatten, Fuse, Rev} For iterators which are generic over allocators it only implements it for the global allocator because we can't conjure an allocator from nothing or would have to turn the allocator field into an `Option` just for this change. These changes will be insta-stable. ACP: https://github.com/rust-lang/libs-team/issues/77