Add enum support

[hngenc]

This PR adds enum support to PeekPokeTesters and AdvTesters by creating a new Pokeable typeclass which we use instead of Bits.

Related issue: chipsalliance/chisel#932

[chick]

This looks good to me

[ucbjrl]

Unfortunately, this currently breaks DspTester in dsptools:

[error] .../dsptools/src/main/scala/dsptools/tester/DspTester.scala:229:5: overloaded method value expect with alternatives:
[error]   (data: dsptools.numbers.DspComplex[_],expected: breeze.math.Complex)Boolean <and>
[error]   (data: chisel3.Data,expected: Double)Boolean <and>
[error]   (signal: chisel3.experimental.FixedPoint,expected: Double)Boolean <and>
[error]   (signal: chisel3.experimental.FixedPoint,expected: Int)Boolean <and>
[error]   (signal: chisel3.SInt,expected: Int)Boolean <and>
[error]   (signal: chisel3.UInt,expected: Int)Boolean <and>
[error]   (signal: chisel3.Bool,expected: Boolean)Boolean <and>
[error]   (signal: chisel3.Bits,expected: BigInt,msg: => String)Boolean <and>
[error]   (good: Boolean,msg: => String)Boolean <and>
[error]   (signal: chisel3.Bundle,expected: Map[String,BigInt])Boolean <and>
[error]   (signal: chisel3.core.Aggregate,expected: IndexedSeq[BigInt])Boolean <and>
[error]   [T <: chisel3.core.Element](signal: T, expected: BigInt, msg: => String)(implicit evidence$14: chisel3.iotesters.Pokeable[T])Boolean
[error]  cannot be applied to (chisel3.Bits, scala.math.BigInt)
[error]     expect(signal.asInstanceOf[Bits], if (expected) BigInt(1) else BigInt(0))
[error]     ^
[error] one error found
[error] (Compile / compileIncremental) Compilation failed
[error] Total time: 33 s, completed Dec 10, 2018 9:29:53 AM

[ucbjrl]

Looks like it might be an easily fixable bug in dsptools.