Vec considered harmful; use isOneOf instead (#64)

Vec is heavyweight and really should only be used for I/O and
dynamic indexing.  A recurring pattern in uncore is

    Vec(const1, const2, const3) contains x

which is nice but has a deleterious effect on simulation copilation
and execution time.  This patch proposes an alternative:

    x isOneOf (const1, const2, const3)
    x isOneOf seqOfThings

I think it's also more idiomatic.

This is just a prototype; I'm not wed to the name or implementation.

uncore/src/main/scala/Util.scala

@@ -0,0 +1,11 @@
+package uncore
+
+import Chisel._
+
+package object Util {
+  implicit class UIntIsOneOf(val x: UInt) extends AnyVal {
+    def isOneOf(s: Seq[UInt]): Bool = s.map(x === _).reduce(_||_)
+  
+    def isOneOf(u1: UInt, u2: UInt*): Bool = isOneOf(u1 +: u2.toSeq)
+  }
+}