support underscore in relative effects

[lrytz]

recognize anonymous function expressions as relative effects. all of the following should work (hopefully - or is an explicit parameter type required?):

def m(f: Int => Int): Int @pure(f.apply(_)) = f(1)
def m(f: Int => Int): Int @pure(f.apply _) = f(1)
def m(f: Int => Int): Int @pure(x => f.apply(x)) = f(1)