Relax type equality constraint to subtype

[travisbrown]

Requiring type equality for syntax methods when there's a constraint on the instance type causes problems if the target type needs to be inferred (see this issue in Cats for some discussion).

Relaxing the constraint to only require evidence of a subtype relationship helps type inference and (as far as I can tell) doesn't cause other problems.

I've also tested this change against 0.6.1 and it looks like it could be backported for a 0.6.2 release (which would help us out with Cats 0.4.0).

/cc @ceedubs @mpilquist

[ceedubs]

👍 looks good to me.

It would probably be nice to have a unit test for the case that we ran into with Cats. But I'm not volunteering to write it right at the moment so this is probably not a very helpful recommendation :P

[travisbrown]

@ceedubs That's reasonable. I just added a test and confirmed that it fails (or rather doesn't compile) without the change in the first commit.

[ceedubs]

@travisbrown fantastic. Thanks!