Similar reasoning: we don't even want to look where the iterator is
pointing if the last call to dbus_signature_iter_next indicated "no more".

In newer versions of libdbus, calling dbus_signature_iter_get_current_type
when the iterator is pointing at the ')' of a struct trips an assertion
failure, rather than just returning INVALID.

This avoids (potentially fatal) warnings, with newer libdbus; it'll only
work if we were compiled against libdbus >= 1.3.0.

* Use a fixed-size struct for String (unicode objects are in fact fixed-size)
  and store its variant_level that way.
* Don't store Struct, _LongBase, _StrBase variant_level and Struct signature
  in a __dict__, but instead have a global dict mapping object IDs to variant
  levels, and a global dict mapping Struct IDs to signatures. This is a bit
  strange, but easier than correctly freeing the __dict__ (which is stored
  at the end of a variable-length struct, so somewhat hard to get at).
* With this change, allocating objects in a loop no longer leaks memory, and
  neither does the test case supplied by Luka Renko.