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/V3Cast.cpp

@@ -16,9 +16,9 @@
 // V3Cast's Transformations:
 //
 // Each module:
-//      For each math operator, if above operator requires 32 bits,
+//      For each expression, if above expression requires 32 bits,
 //      and this isn't, cast to 32 bits.
-//      Likewise for 64 bit operators.
+//      Likewise for 64 bit expressions.
 //
 // C++ rules:
 //      Integral promotions allow conversion to larger int.  Unsigned is only
@@ -158,7 +158,8 @@ private:
     }
     void visit(AstVarRef* nodep) override {
         const AstNode* const backp = nodep->backp();
-        if (nodep->access().isReadOnly() && !VN_IS(backp, CCast) && VN_IS(backp, NodeMath)
+        if (nodep->access().isReadOnly() && VN_IS(backp, NodeExpr) && !VN_IS(backp, CCast)
+            && !VN_IS(backp, NodeCCall) && !VN_IS(backp, CMethodHard) && !VN_IS(backp, SFormatF)
             && !VN_IS(backp, ArraySel) && !VN_IS(backp, RedXor)
             && (nodep->varp()->basicp() && !nodep->varp()->basicp()->isTriggerVec()
                 && !nodep->varp()->basicp()->isForkSync())