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/V3LinkInc.cpp

@@ -175,10 +175,6 @@ private:
         m_insStmtp = nullptr;
     }
     void visit(AstNodeStmt* nodep) override {
-        if (!nodep->isStatement()) {
-            iterateChildren(nodep);
-            return;
-        }
         m_insMode = IM_BEFORE;
         m_insStmtp = nodep;
         iterateChildren(nodep);