ChiselSim peek/poke/expect for Enums, Records, Vecs, and generic Data

[kammoh]

This PR changes the "testable" implicit classes in PeekPokeAPI to allow for a unified peek(), poke(), and expect API for all data types, including Records, Vecs, Enums, and nested aggregates. This unified API would be very helpful when developing higher-level test abstractions, such as enqueue/deque over clocked Decoupled interfaces.

This is a step towards adding some useful features and ergonomic APIs similar to those provided by chiseltest (#4209 included a rough PoC for integrating some of these features).

The code does not look very pretty, but I could not figure out a cleaner way to make it work. Also, there is some repeated code.

I'd greatly appreciate comments and ideas for an alternative approach or other improvements.

Note: The previous (existing) implementation of expect[T]() seems incorrect. In addition to mistakingly using the same type name, T for the different method type parameter, the comparison uses object equality (!=).

def expect[T](
      expected:     T,
      encode:       (Simulation.Value) => T,
      buildMessage: (T, T) => String,
      sourceInfo:   SourceInfo
    ): Unit = {
...
        if (observed != expected) {

For the implemented Element values, the observed value is tuned into a Chisel object using the encode() method. Since the objects are not the same, the comparison would always fail, even when the value is expected. Seems the previous tests did not cover this, but using the tests in this PR, I get false failures with the previous expect + encode:

Observed value: 'UInt<33>(3521158774)'
Expected value: 'UInt<33>(3521158774)'

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