* Sniff the first byte to glean if the incoming request is a single or batch request

This works around the serde limitations around `untagged` enums and `RawValue`.

* fmt

* For http server, check first byte before allocating space for the body

Also, rework the way we return errors: prefer JSON-RPC errors according to spec (application/json) wherever sensible.

* Review feedback

* Don't assume there is a first byte to read

* ty clipyp

* Review concerns

* Cleanup