`Trace` derive macro fails on some self-referential types

[s9ferech]

I have created a test that demonstrates how the Trace derive macro fails on types with a field that depends on a type parameter and also on the newly defined type itself. In this case, the trait resolution runs into a cycle when it tries to derive a trait instance.

If the field was only self-referential but did not include a type parameter then this would work because synstructure would not include the field in the where clause. If it depended on a type parameter but was not self-referential then there would be no cycle. Only in combination do they cause this failure.