TextMarshaler map keys for sets

This is a minor release that enables map[X]struct{} or map[X]bool to be used as a set, where X implements encoding.TextMarshaler and encoding.TextUnmarshaler.
Additionally, error handling for operations has been standardized to always return the first error they encounter, previously it was inconsistent depending on the operation. Put's error handling also had a bug that caused it to not return pre-run errors like encoding issues, which could result in unhelpful error messages. This is now fixed.