Rename this repo to `nim-json-rpc` and generalize it even further

[zah]

This repo should allow you to create JSON-RPC services that are accessible over different underlying "transports". For example, the Language Server Protocol that will be implemented soon for Nim is also based on JSON-RPC, but it uses a simple line-delimited text stream communicated over stdin/stdout.

From the perspective of this library, the underlying connection-acceptor/stream-creator should be defined either as a run-time interface or as a concept.

[zah]

Another example is that it should be easy to integrate with a WebSocket server hiding the underlying frame-encoding details.

[zah]

There is an effort from @PMunch to create a language server implementation and I've provided the initial steps here:
https://github.com/nim-lang/langserver

After some discussions, it's clear that another feature that might help is having a default RPC handler. It will need to accept the raw JSON and it will be used to log the unsupported calls and to provide default error responses.

The messages in the language server protocol also features a number of optional fields. We can support those by using option[T] as the type of the field - then a null value or a field missing altogether can be interpreted as a none value.

[zah]

The objectives of this issue are not realized through the route API which is transport-agnostic.