[xmonad] change fixity of (-->)

[Adam Vogt]

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