`yampa`: `Yampa.FRP.Task` does not define `return` in terms of `pure`

[ivanperez-keera]

The monad instance for Yampa.FRP.Task.Task does not define return in terms of the Applicative pure. This is leading to a warning due to upcoming changes in GHC:

src/FRP/Yampa/Task.hs:113:3: warning: [-Wnoncanonical-monad-instances]
    Noncanonical `return' definition detected
    in the instance declaration for `Monad (Task a b)'.
    `return' will eventually be removed in favour of `pure'
    Either remove definition for `return' (recommended) or define as `return = pure'
    See also: https://gitlab.haskell.org/ghc/ghc/-/wikis/proposal/monad-of-no-return

The definition is the same as that of pure, so there's no reason not to rely on it.