More natural handler prototypes

Allow the signature of a handler to reflect the incoming data. For example instead of:

```go
func sum(request jsonrpc.RequestResponder) jsonrpc.Response {
total := 0.0
for _, x := range request.Params().([]interface{}) {
total += x.(float64)
}

return request.NewSuccessResponse(total)
}
```

Use:

```go
func sum(nums ...float64) (interface{}, error) {
total := 0.0
for _, x := range nums {
total += x
}

return total, nil
}
```

The issue names no files, tests, or entry points. Start by tracing how handlers are registered and invoked, then compare that path with the two Go signatures shown; done means handlers can receive typed incoming data and return the demonstrated result and error form without the current request-level parsing.

Written by the indexing model from the issue text.