Spike an OpenAPI 3.0-generated Stripe client library

[exarkun]

In #130 we switched to a stripe-core fork. Our fork has changes that are ... not great. It will not be practical to maintain this fork with changes like the ones we're currently carrying. One option is to do some more serious work in stripe-core itself but another option is to find a different way to interact with Stripe.

One option for the latter is to use https://github.com/Haskell-OpenAPI-Code-Generator/Haskell-OpenAPI-Client-Code-Generator to generate a Haskell client library from https://github.com/stripe/openapi.

Spike this to see if it is really feasible or not.