Move chisel3 compatible casts {as,to}{S,U}Int from Bits to Node. (#744)

Make their definitions available to Aggregates.
Add them to Bundle keywords.

Author: ucbjrl

src/main/scala/Bits.scala

@@ -72,14 +72,8 @@ abstract class Bits extends Data with proc {
   }
 
   def fromInt(x: Int): this.type
-  def toSInt(): SInt = chiselCast(this){SInt()}
-  def toUInt(): UInt = chiselCast(this){UInt()}
   override def getNode: Node = if (procAssigned) this else super.getNode
 
-  // Chisel3 - rename these to make the reinterpret cast more explicit
-  final def asUInt() = toUInt
-  final def asSInt() = toSInt
-
   // internal, non user exposed connectors
   private var assigned = false
   private def checkAssign(src: Node) = {

src/main/scala/Bundle.scala

@@ -35,7 +35,7 @@ object Bundle {
   val keywords = Set("elements", "flip", "toString",
     "flatten", "binding", "asInput", "asOutput", "unary_$tilde",
     "unary_$bang", "unary_$minus", "clone", "cloneType", "toUInt", "toBits",
-    "toBool", "toSInt", "asDirectionless")
+    "toBool", "toSInt", "asDirectionless", "asUInt", "asSInt")
 
   def apply[T <: Bundle](b: => T)(implicit p: Parameters): T = {
     Driver.parStack.push(p.push)

src/main/scala/Node.scala

@@ -439,9 +439,14 @@ abstract class Node extends Nameable {
     if (!isTypeNode || inputs.isEmpty) this
     else inputs(0).getNode
 
-  // TODO: change function name to toUInt?
   /** @return This node as a UInt */
   def toBits(): UInt = chiselCast(this){UInt()}
+  def toSInt(): SInt = chiselCast(this){SInt()}
+  def toUInt(): UInt = chiselCast(this){UInt()}
+
+  // Chisel3 - rename these to make the reinterpret cast more explicit
+  final def asUInt() = toUInt()
+  final def asSInt() = toSInt()
 
   /** @return This node */
   def toNode: Node = this