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/V3Randomize.cpp

@@ -172,7 +172,7 @@ private:
             }
             return stmtsp;
         } else {
-            AstNodeMath* valp;
+            AstNodeExpr* valp;
             if (auto* const enumDtp = VN_CAST(memberp ? memberp->subDTypep()->subDTypep()
                                                       : varrefp->dtypep()->subDTypep(),
                                               EnumDType)) {