contrib: removed 'all(..)' since it's now in core, added 'race(..)' plugin since it's not actually an alias of 'gate(..)', added 'rejectAfter(..)' plugin, refactored 'first(..)', 'last(..)', 'any(..)', and 'none(..)' to be able to accept sequences directly, refactored 'iterable(..)' to use exported-internal 'schedule(..)' from core, refactored to simplify 'errfcb()' plugin