[chiselsim] Add SimulatorAPI, reduce DefaultSimulator to nothing

[seldridge]

Add a new SimulatorAPI trait which provides methods for running
simulations. This will submsume the existing DefaultSimulator.simulate
method and extends it with an "ABI" for how to startup simulations of
Chisel-generated Verilog.

This API includes two methods:

1. simulateRaw -- This is only usable with RawModules where there is
   no information about the clock and reset. This is the same as
   DefaultSimulator.simulate.

2. simulate -- This is only usable with Modules where there is a
   defined clock and reset. This is the method which implements an "ABI"
   startup sequence which avoids problems with simulators, time zero
   behavior, and race conditions.

Remove all methods from DefaultSimulator and replace them with methods
provided by the SimulatorAPI. This is change to the unreleased
simulate method for DefaultSimulator in that this will now require a
Module and uses the standard ABI bringup procedure.
EphemeralSimulator is left unchanged.

Release Notes

Add a new ChiselSim trait, SimulatorAPI, which adds methods for running simulations.

[seldridge]

@jackkoenig: I'm curious if you have any ideas on how to package up this API. Two options occurred to me:

1. Just do trait mix-in:

class Foo extneds AnyFunSpec with Matchers with SimulatorAPI with PeekPokeAPI {
  /** body */
}

2. Wildcard import pattern:

import chisel3.simulator.DefaultSimulator._

If (2), then what should the package/object be?

3. Something else?

I'd plan to keep EphemeralSimulator around, but deprecate it.

[seldridge]

Maybe for (3) we call this ChiselSim. I.e., "ChiselSim" is just the collection of APIs that you use for running simulations. I'd then propose two slightly different packagings of it:

package chisel3.simulator

trait ChiselSim extends PeekPokeAPI with SimulatorAPI

package chisel3.simulator.scalatest

import chisel3.simulator.{PeekPokeAPI, SimulatorAPI}

trait ChiselSim extends PeekPokeAPI with SimulatorAPI with WithTestingDirectory /** with other ScalaTest stuff */

Note: this can either or instead use the wildcard import approach with some slight changes to how WithTestingDirectory exposes its implicit in an object instead of in a trait.