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/V3ActiveTop.cpp

@@ -129,7 +129,7 @@ class ActiveTopVisitor final : public VNVisitor {
         nodep->v3fatalSrc("Node should have been under ACTIVE");
     }
     //--------------------
-    void visit(AstNodeMath*) override {}  // Accelerate
+    void visit(AstNodeExpr*) override {}  // Accelerate
     void visit(AstVarScope*) override {}  // Accelerate
     void visit(AstNode* nodep) override { iterateChildren(nodep); }