Internals: Move the eval loop into the runtime library (#8225)

The loops modelling the SystemVerilog scheduling regions are no longer
generated. They now live in 'VerilatedEvalLoop' in the runtime library.
The generated model holds one as a member, passing itself to it, and
exposes each evaluation entry point to it as a pure virtual method on
VerilatedModel. The model's 'eval' and 'eval_step' remain the top level
entry points, and are backward compatible.

V3Sched no longer emits '_eval' or '_eval_settle', etc.. Instead every
evaluation entry point called from the runtime is enumerated by 'VEval',

Scheduling creates all entry points, for all scheduling regions, even if
they are empty, and the runtime eval loop calls everything
unconditionally. If regions are empty, this is simply a call to an empty
function. This will hurt performance on very small models, but should
not be noticeable on anything meaningful, so it is likely best to keep
to reduce complexity.

A scheduling entry points evaluate a single iteration and returns
whether it did any work, they are effectively the previous
`_eval_phase_*` functions.

src/Verilator.cpp

@@ -587,10 +587,6 @@ static void process() {
             }
         }
 
-        // These are no longer needed, remove references before CFunc inlining
-        v3Global.rootp()->evalp(nullptr);
-        v3Global.rootp()->evalNbap(nullptr);
-
         if (!v3Global.opt.lintOnly() && !v3Global.opt.serializeOnly()) {
             if (v3Global.opt.fInlineCFuncs()) {
                 // Inline small CFuncs to reduce function call overhead