This was dropped last-minute from d94de87252d0fe2ae97341d186b03a2fbe136b04
together with the default implementation for (>>)
(see 65f887e1a0d864526f6a2609a3afc2c151c25e38 for explanation).
However, the risk of accidental mutually recursive definitions of
return/pure is rather low, while not having the default method
implementation in place as soon as possible would complicate
any future attempt to clean-up the Monad-class.