[RTL] Move rtl.module outputs to be "result like"

[lattner]

Currently outputs to rtl.module are represented as function/bb arguments that are connected to. We should move to a mode where they are more "result like". For example, instead of:

 rtl.module @C(%a: i1 {rtl.direction = "in"},
                %b: i1 {rtl.direction = "out"}) {
    %0 = rtl.shl %a, %a : i1
    rtl.connect %b, %0 : i1
  }

we should have something like:

 rtl.module @C(%a: i1 {rtl.direction = "in"}) -> "b": i1 {
    %0 = rtl.shl %a, %a : i1
    return %0 : i1
  }

Given this, we can drop the rtl.direction attribute, changing it to something like an rtl.inout nullary attribute to represent the inout case.