Make all expressions derive from AstNodeExpr (#3721).

Apart from the representational changes below, this patch renames
AstNodeMath to AstNodeExpr, and AstCMath to AstCExpr.

Now every expression (i.e.: those AstNodes that represent a [possibly
void] value, with value being interpreted in a very general sense) has
AstNodeExpr as a super class. This necessitates the introduction of an
AstStmtExpr, which represents an expression in statement position, e.g :
'foo();' would be represented as AstStmtExpr(AstCCall(foo)). In exchange
we can get rid of isStatement() in AstNodeStmt, which now really always
represent a statement

Peak memory consumption and verilation speed are not measurably changed.

Partial step towards #3420

src/V3Partition.cpp

@@ -3186,8 +3186,9 @@ static void addThreadStartToExecGraph(AstExecGraph* const execGraphp,
         } else {
             // The last will run on the main thread.
             AstCCall* const callp = new AstCCall(fl, funcp);
+            callp->dtypeSetVoid();
             callp->argTypes("vlSelf, vlSymsp->__Vm_even_cycle__" + tag);
-            execGraphp->addStmtsp(callp);
+            execGraphp->addStmtsp(callp->makeStmt());
             addStrStmt("Verilated::mtaskId(0);\n");
         }
     }