POST/PUT/DELETE body parsing

Currently `POST` `PUT` and `DELETE` requests are registered and can be handled, but the data isn't being parsed in any way that is usable.

Either implement a built in body parsing utility or allow uses to implement existing implementations such as `body-parser` from Express.

Start by tracing how POST, PUT, and DELETE requests are registered and handled, then compare whether an Express body-parser integration or a built-in utility fits the stated goal. Done means request bodies are parsed into a usable form for these methods, with the chosen approach verified.

Written by the indexing model from the issue text.