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/V3Delayed.cpp

@@ -485,9 +485,8 @@ private:
                 AstIf* const ifp = new AstIf{flp, dlyRef(VAccess::READ)};
                 postp->addStmtsp(ifp);
                 AstCMethodHard* const callp = new AstCMethodHard{flp, vrefp, "fire"};
-                callp->statement(true);
                 callp->dtypeSetVoid();
-                ifp->addThensp(callp);
+                ifp->addThensp(callp->makeStmt());
             }
 
             AstActive* const activep = createActive(nodep);
@@ -501,8 +500,7 @@ private:
             AstCMethodHard* const callp
                 = new AstCMethodHard{flp, nodep->operandp()->unlinkFrBack(), "fire"};
             callp->dtypeSetVoid();
-            callp->statement(true);
-            nodep->replaceWith(callp);
+            nodep->replaceWith(callp->makeStmt());
         }
         nodep->deleteTree();
     }