Patch to make need/want deal without lists

[Neil Mitchell]

Hi,

A patch has been submitted that makes need/want generalised so they
can work with either [FilePath] or FilePath. See
https://github.com/ndmitchell/shake/pull/180

Basically it introduces a class Targets, so:

-need :: [FilePath] -> Action ()
+need :: Targets targets => targets -> Action ()

* Good: Uses look better, no more need ["foo"], it's just need "foo"
* Good: Flexible to add Maybe or ByteString/Text and other types of path.
* Bad: More complex type signature since it's more general
* Bad: It's less obvious that need requires a list for parallelism
(but if it did matter seriously, I could change the monad to defer
needs a bit longer and group them again)

I'm on the fence, so if any Shake users have opinions, please share
them here or on the GitHub ticket.

Thanks, Neil