Support JsValue's inside Result and Option types

[allsey87]

The arguments of RPC currently have to be either serializable or JsValue. Ideally, this restriction should be relaxed to allow composite types, e.g., an arbitrary struct or enum containing a JsValue to be sent across a channel, however, at a bare minimum it should be possible to have RPC calls that can return a Result<T, JsValue> since this pattern is used to represent exceptions from Javascript code.

[allsey87]

Note that only some errors are supported by the structured clone algorithm: https://developer.mozilla.org/en-US/docs/Web/API/Web_Workers_API/Structured_clone_algorithm#javascript_types