Use express hooks to custom serialize objects

[JaneJeon]

Part of the reason why fastify is fast is that it relies on fast-json-stringify to stringify its outputs. And we already use jsonschema for everything, so integration with fast-json-stringify might be possible by hooking into res.send():

https://github.com/expressjs/express/blob/0a48e18056865364b2461b2ece7ccb2d1075d3c9/lib/response.js#L1118

Of course, this goes against #54 if we decide to go with this approach instead.