sim: split into base, core, and engines.

Before this commit, each simulation engine (which is only pysim at
the moment, but also cxxsim soon) was a subclass of SimulatorCore,
and every simulation engine module would essentially duplicate
the complete structure of a simulator, with code partially shared.

This was a really bad idea: it was inconvenient to use, with
downstream code having to branch between e.g. PySettle and CxxSettle;
it had no well-defined external interface; it had multiple virtually
identical entry points; and it had no separation between simulation
algorithms and glue code.

This commit completely rearranges simulation code.
  1. sim._base defines internal simulation interfaces. The clarity of
     these internal interfaces is important because simulation
     engines mix and match components to provide a consistent API
     regardless of the chosen engine.
  2. sim.core defines the external simulation interface: the commands
     and the simulator facade. The facade provides a single entry
     point and, when possible, validates or lowers user input.
     It also imports built-in simulation engines by their symbolic
     name, avoiding eager imports of pyvcd or ctypes.
  3. sim.xxxsim (currently, only sim.pysim) defines the simulator
     implementation: time and state management, process scheduling,
     and waveform dumping.

The new simulator structure has none of the downsides of the old one.

See #324.

nmigen/vendor/lattice_machxo2.py

@@ -7,5 +7,5 @@ __all__ = ["LatticeMachXO2Platform"]
 
 
 # TODO(nmigen-0.4): remove
-warnings.warn("instead of vendor.lattice_machxo2, use vendor.lattice_machxo_2_3l",
+warnings.warn("instead of nmigen.vendor.lattice_machxo2, use nmigen.vendor.lattice_machxo_2_3l",
               DeprecationWarning, stacklevel=2)