proposal #2517: remove 'pure' method from Arrow class

[Ross Paterson]

The Arrow class as originally defined by John Hughes had methods arr, >>>
and first.  (>>> has since been moved by #1773 to the Category class.)
When writing the "Fun of Programming" paper, I added pure as a synonym
for arr, because Richard Bird preferred it.  However this name hasn't
caught on, and now it clashes with a method in the Applicative class,
so I propose to remove it.

The usual practice would be to deprecate the name in one release and
remove it in the following one, but I propose to remove it in one step
because
 * no-one seems to be using this name, and
 * backward compatibility has been broken anyway by the Category split
   (#1773).
The only people who will be bitten by the change are those who import
Control.Arrow hiding pure, and they wouldn't be warned by deprecation.