* 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.

This avoids us getting kicked off the bus when trying to put the wrong
number of things in a struct - this used to happen, but was masked by the fact
that the tests ran with service activation, so the service was just killed and
reactivated. Forthcoming changes to get_object make this automatic reactivation
not happen (messages will be directed to the unique name by default, so
stateful communication can work).