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/V3Depth.cpp

@@ -107,16 +107,10 @@ private:
             iterateChildren(nodep);
         }
     }
-    void visit(AstNodeStmt* nodep) override {
-        if (!nodep->isStatement()) {
-            iterateChildren(nodep);
-        } else {
-            visitStmt(nodep);
-        }
-    }
+    void visit(AstNodeStmt* nodep) override { visitStmt(nodep); }
     // Operators
     void visit(AstNodeTermop* nodep) override {}
-    void visit(AstNodeMath* nodep) override {
+    void visit(AstNodeExpr* nodep) override {
         // We have some operator defines that use 2 parens, so += 2.
         {
             VL_RESTORER(m_depth);