Split jsonrpc2.go file into multiple files (#65)

This merge request moves some of the contents from the jsonrpc2.go file
into their own designated file. The new files being introduced
(excluding test files) are as follows:

* conn.go
* request.go
* response.go

The motive of this change is to make it easier to navigate the code.
Without this change, the jsonrpc2.go file is 813 lines of code.

response.go

@@ -0,0 +1,72 @@
+package jsonrpc2
+
+import (
+	"encoding/json"
+	"errors"
+)
+
+// Response represents a JSON-RPC response. See
+// http://www.jsonrpc.org/specification#response_object.
+type Response struct {
+	ID     ID               `json:"id"`
+	Result *json.RawMessage `json:"result,omitempty"`
+	Error  *Error           `json:"error,omitempty"`
+
+	// Meta optionally provides metadata to include in the response.
+	//
+	// NOTE: It is not part of spec. However, it is useful for propagating
+	// tracing context, etc.
+	Meta *json.RawMessage `json:"meta,omitempty"`
+
+	// SPEC NOTE: The spec says "If there was an error in detecting
+	// the id in the Request object (e.g. Parse error/Invalid
+	// Request), it MUST be Null." If we made the ID field nullable,
+	// then we'd have to make it a pointer type. For simplicity, we're
+	// ignoring the case where there was an error in detecting the ID
+	// in the Request object.
+}
+
+// MarshalJSON implements json.Marshaler and adds the "jsonrpc":"2.0"
+// property.
+func (r Response) MarshalJSON() ([]byte, error) {
+	if (r.Result == nil || len(*r.Result) == 0) && r.Error == nil {
+		return nil, errors.New("can't marshal *jsonrpc2.Response (must have result or error)")
+	}
+	type tmpType Response // avoid infinite MarshalJSON recursion
+	b, err := json.Marshal(tmpType(r))
+	if err != nil {
+		return nil, err
+	}
+	b = append(b[:len(b)-1], []byte(`,"jsonrpc":"2.0"}`)...)
+	return b, nil
+}
+
+// UnmarshalJSON implements json.Unmarshaler.
+func (r *Response) UnmarshalJSON(data []byte) error {
+	type tmpType Response
+
+	// Detect if the "result" field is JSON "null" or just not present
+	// by seeing if the field gets overwritten to nil.
+	*r = Response{Result: &json.RawMessage{}}
+
+	if err := json.Unmarshal(data, (*tmpType)(r)); err != nil {
+		return err
+	}
+	if r.Result == nil { // JSON "null"
+		r.Result = &jsonNull
+	} else if len(*r.Result) == 0 {
+		r.Result = nil
+	}
+	return nil
+}
+
+// SetResult sets r.Result to the JSON representation of v. If JSON
+// marshaling fails, it returns an error.
+func (r *Response) SetResult(v interface{}) error {
+	b, err := json.Marshal(v)
+	if err != nil {
+		return err
+	}
+	r.Result = (*json.RawMessage)(&b)
+	return nil
+}