Properly decode partialOperationContext in all hooks and components. Update api.md to reflect latest changes to the reason-urql API.

[parkerziegler]

This is the last PR before we release v1.5.0 of reason-urql 🎉 The core change here is that we properly decode a partialOperationContext into a format that urql can understand. The key part here is that we expose requestPolicy as a polymorphic variant on the Reason side, so it needs to be properly decoded into its JS string representation before it can be sent to urql.

I also updated the api.md file to more accurately reflect the current state of the API. That was in a bit of rough shape, so I want to continue improving it over the coming releases, or at least break it up into more manageable chunks.

This will be the last release we do before the migration to BuckleScript 7! That's going to get rid of so much of the [@bs.deriving abstract] nonsense we currently have to do. Pure record to object support will be so great! That, alongside a migration to urql 1.4.0, will form the basis of either 1.6.0 (if changes to the API are minimal) or v2 (if they're more involved).