Add `instrospect.contains()` and string to comptr

[s-hadinger]

Add two features to import introspect:

• instrospect.contains( instance or module or class, name:string) -> bool: returns true if the instance/class/module contains a member with name
• introspect.toptr() now works also for a string and returns a pointer to the C string (warning, the pointer is only valid until next GC)