Merlin-destruct can't handle some GADT types

[ncik-roberts]

See #36 for the test demonstrating as such, and #38 for the sad workaround.

[goldfirere]

Has this been reported upstream? I'd be surprised if the problem doesn't also exist there.

[ncik-roberts]

I could not reproduce the issue upstream. (So, while an analogous code issue could well exist upstream, it's not in a state where we can make a GitHub issue.)