#[serde(untagged)] doesn't work with RawValue

[dvdplm]

For reasons outlined here, RawValue can't be
used with untagged enums at the moment.
This means we can't use an SingleOrBatch untagged enum to handle incoming requests that can be one of:

• single JSONRPC request, i.e. {"jsonrpc":"2.0","method":"bar","id":1}
• a batch of requests, i.e. [{"jsonrpc":"2.0","method":"baz","id":1}, {"jsonrpc":"2.0","method":"baz","id":2}]
• a single notification, i.e. {"jsonrpc":"2.0","method":"bezz"}
• a batch of notifications, i.e. [{"jsonrpc":"2.0","method":"bezz"}, {"jsonrpc":"2.0","method":"bozz"}, {"jsonrpc":"2.0","method":"bizz"}]

(We do not support notifications, but if we ever were to do that to be fully compliant with the spec, this problem would make things worse)

Instead we have to try each case individually: first the single request case, then the batch case and then one time more to check for the error. For the worst case – unparseable input – we make three calls to [serde_json::from_slice] which is pretty annoying.

There is a PoC fix for this in serde-rs/serde#1354

Suggestion: revisit this when/if serde-rs/serde#1354 lands.

[dvdplm]

A possible workaround here is to do what @maciejhirsz did with the optional params parsing: check the first byte for a [ (batch).