Introduces Textpattern_Type_Callable.

Converts callables to string presentation. Turn
callback_tostring to a wrapper. Extends the conversion
support to Closures and other objects.

git-svn-id: http://textpattern.googlecode.com/svn/development/4.x@5651 2fea6a4d-a838-0410-917b-93a53c48e9d2