Fastify integration
[Fastify](https://github.com/fastify/fastify) has almost 11,000 stars on github, over 50,000 weekly downloads, is depended upon by 240 packages and claims to be [very fast](https://github.com/fastify/fastify#benchmarks).

I'd like to see it supported, hopefully pitch in.

Perhaps we could envisage splitting up the modules (grant and grant-profile) according consumers, but that would be a breaking change, and topic for another issue. Let me know if I should open that too.