Add simulation context (VerilatedContext) (#2660). (#2813)

**   Add simulation context (VerilatedContext) to allow multiple fully independent
      models to be in the same process.  Please see the updated examples.
**   Add context->time() and context->timeInc() API calls, to set simulation time.
      These now are recommended in place of the legacy sc_time_stamp().

src/V3Assert.cpp

@@ -96,7 +96,8 @@ private:
                    :  // If assertions are off, have constant propagation rip them out later
                  // This allows syntax errors and such to be detected normally.
                  (v3Global.opt.assertOn()
-                      ? static_cast<AstNode*>(new AstCMath(fl, "Verilated::assertOn()", 1))
+                      ? static_cast<AstNode*>(
+                          new AstCMath(fl, "vlSymsp->_vm_contextp__->assertOn()", 1))
                       : static_cast<AstNode*>(new AstConst(fl, AstConst::BitFalse())))),
             nodep, nullptr);
         newp->user1(true);  // Don't assert/cover this if