Improve Maybe API

Maybe monad should be close to Haskell or Scala's API
=> #2

Add methods:
to_s, inspect: for easy-to-read
nothing?, just?: instead of failure? and success?
and remove following methods:

failure, success?: due to the above reasons.
#== is implemented for each Just and Nothing and remove Maybe's
=> #3

Remove remained old 'equal?' methods in Nothing
=> #4