@to_yojson and @of_yojson attributes to support custom serde impls

[cemerick]

(Addressing #106.)

This works for me, and tests clean under 4.04 and 4.07.

I think the only set of cases that aren't supported here are variants, the existing implementation of which depends on having a fixed representation in multiple places. It might be good and worthwhile to support these new attrs for individual variants, but it seems reasonable to address that separately?

FYI, this is my first time working with a ppx's implementation, so bear that in mind upon review. 😊

[cemerick]

Forgot to mention: I'll add notes/docs as appropriate prior to merging if what's here is 👍

[whitequark]

I'll add notes/docs as appropriate prior to merging if what's here is

Ah sorry, missed that. Feel free to add docs as a separate PR.

[cemerick]

No worries, will get something in later this morning.