_pygi_marshal_to_py_interface_struct() and other places treat subtypes of
G_TYPE_BOXED as boxed values and wrap them with _pygi_boxed_new(). Fix
_caller_alloc() and _cleanup_caller_allocates() to consider G_TYPE_BOXED
subtypes as well and use the slice allocator instead of malloc()'ing a struct.
This avoids trying to free an malloc'ed struct with g_slice_free() and properly
cleans up the boxed values.

The leak was produced with:

G_SLICE=debug-blocks PYTHONPATH=. valgrind --tool=memcheck --leak-check=full --show-possibly-lost=no \
  python3 -c 'from gi.repository import Gtk; b=Gtk.TextBuffer(); (s,e) = b.get_bounds()'