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/V3Hasher.cpp

@@ -183,8 +183,8 @@ private:
     }
 
     //------------------------------------------------------------
-    // AstNodeMath
-    void visit(AstNodeMath* nodep) override {
+    // AstNodeExpr
+    void visit(AstNodeExpr* nodep) override {
         m_hash += hashNodeAndIterate(nodep, HASH_DTYPE, HASH_CHILDREN, [=]() {});
     }
     void visit(AstConst* nodep) override {