Remove unnecessary calls to iter

On Py3, dict.items, et. al return iterators. Calling iter
over these methods only served to slow things down for
marshmallow's use cases
Add test for storing multiple errors when many=True

along with other small miscellanea
Update docs
Update docs
Add aliases for common fields
Merge branch 'feat/aliases' into dev
Fix UUID deserialization
Store additional Field kwargs as metadata
Add aliases to __all__