Type signatures in do bindings classed as top level definitions, removing (<-) highlighting

[tomsmalley]

This behaviour is not observed when the part to the left of (<-) is wrapped in parenthesis.

[JustusAdam]

Should be easy to fix. Look for the section where type signatures are defined (search for the :: in the grammar). And there is a bunch of patterns defined for when it should stop (such as } and )) just add <- and that'll probably do it.