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/V3Order.cpp

@@ -1242,9 +1242,10 @@ AstActive* OrderProcess::processMoveOneLogic(const OrderLogicVertex* lvertexp,
             scopep->addBlocksp(newFuncpr);
             // Create top call to it
             AstCCall* const callp = new AstCCall{nodep->fileline(), newFuncpr};
+            callp->dtypeSetVoid();
             // Where will we be adding the call?
             AstActive* const newActivep = new AstActive{nodep->fileline(), name, domainp};
-            newActivep->addStmtsp(callp);
+            newActivep->addStmtsp(callp->makeStmt());
             if (!activep) {
                 activep = newActivep;
             } else {