std: Rename Iterator adaptor types to drop the -Iterator suffix

Drop the "Iterator" suffix for the the structs in std::iterator.
Filter, Zip, Chain etc. are shorter type names for when iterator
pipelines need their types written out in full in return value types, so
it's easier to read and write. the iterator module already forms enough
namespace.
auto merge of #8090 : blake2-ppc/rust/iterator-adaptor-names, r=pcwalton

Drop the "Iterator" suffix for the the structs in std::iterator.
Filter, Zip, Chain etc. are shorter type names for when iterator
pipelines need their types written out in full in return value types, so
it's easier to read and write. the iterator module already forms enough
namespace.