Add support for multi-element vector swizzles. Issue #17.

This commit adds support for swizzles like "foo.zy" (if "foo" is,
for example, a float<3> type) as rvalues.  (Still need support for
swizzles as lvalues.)

parse.yy

@@ -269,7 +269,7 @@ postfix_expression
     | TOKEN_LAUNCH '<' postfix_expression '(' ')' '>'
       { $$ = new FunctionCallExpr($3, new ExprList(@3), @3, true); }
     | postfix_expression '.' TOKEN_IDENTIFIER
-      { $$ = new MemberExpr($1, yytext, @1, @3); }
+      { $$ = MemberExpr::create($1, yytext, @1, @3); }
 /*    | postfix_expression TOKEN_PTR_OP TOKEN_IDENTIFIER
       { UNIMPLEMENTED }
 */