[xmonad] change fixity of (-->)

Hi, Writing complicated managehooks requires parentheses at the moment, since only (<&&>) has a fixity specified (infixr 3):

(appName ?= x <&&> classname ?= y) --> (doFloat >> doIgnore)

I propose the addition:

infix 0 -->

appName ?= x <&&> classname ?= y --> doFloat >> doIgnore

I can't think of when this would break a working ocnfig, since current managehooks are either simple, or explicitly parenthesized already. Thanks, Adam