Handle recursive enum

[tyt2y3]

First of all thank you for such an awesome crate. It's used in sea-query https://github.com/SeaQL/sea-query/blob/f4a5172b9493f355567ee01613e05b2886ed2673/src/value.rs#L132

I discovered upon 0.5.12 the code fails to compile, presumably due to the change introduced in 'ijackson-precise-bounds'.

Here is the testcase https://github.com/magiclen/educe/pull/26

May be recursive types is an undocumented feature that we somehow depended on? Or is this a bug introduced during refactoring?

Would love to hear your suggestions.

Ref: https://github.com/SeaQL/sea-query/issues/781 (to reproduce there do cargo b --features=hashable-value,postgres-array

[sfackler]

This regression appears to be present in 0.6.0 as well.