Fix IPC handling of jsonrpc errors

[oliver-giersch]

Motivation

Fixes #1119: JSON-RPC error responses are not propagated to (request) callsites for the IPC transport correctly and end up as unhelpful serde_json errors.

Tangentially, the internal handling of all JSON values creates a lot of intermediate serde_json::Value instances, which is inefficient, since this creates potentially nested HashMaps.

Solution

Instead of calling Response::to_value, which converts both success and error responses into a serde_json::Value we call Response::into_result, which propagates error responses as Err variants.

Internal handling of JSON responses and requests is switched from serde_json::Value to serde_json::value::RawValue, which does not create any hashmaps.

Not Solved

A potential issue persists, in that internal IPC server errors (such as an unexpected disconnect of the IPC socket, e.g. a node crash) are not propagated to pending subscriptions, for other pending requests, this should be correctly propagated now, but for subscriptions this would require a change to the PubSubClient trait definition (i.e., sending Result notifications).

[mattsse]

this looks great,
propagating error responses as Err variants is more useable.

probably missing cargo +nightly fmt

[gakonst]

Nice work!