Improved Cycle Detection Logic

[InsertCreativityHere]

This PR fixes and improves the cycle detection logic.

1. Currently, the detector doesn't check inside of sequences, dictionaries, or results, so struct S { s: Sequence<S> } won't be caught.
   This PR improves the detector to check these kinds of things.

2. Adds a path for the detector to check enums with fields for cycles.
   (Currently it does not).

3. It adds logic to avoid outputting duplicate cycles:
   A -> B -> A and B -> A -> B are really the same cycle, no point in outputting both.
   We avoid duplicates by storing sets of cycle elements like {A, B}.
   If we detect a cycle, and it's set of elements has already been reported, we don't report it again.

Fixes #683.

[externl]

Looks good!