Remove support for parameter injection. (#82)

This feature adds a lot of code for relatively little benefit. In practice, a
service can achieve the same effect by embedding common headers in its argument
types, which gives more control to both client and server without the overhead
of an additional encoding step.

- Remove the EncodeContext and DecodeContext options.
- Remove the plumbing to encode (client) and decode (server) contexts.
- Remove the jctx package.
- Update usage in the jcall command-line tool.
- Clean up tests.

bench_test.go

@@ -10,7 +10,6 @@ import (
 
 	"github.com/creachadair/jrpc2"
 	"github.com/creachadair/jrpc2/handler"
-	"github.com/creachadair/jrpc2/jctx"
 	"github.com/creachadair/jrpc2/server"
 	"github.com/fortytw2/leaktest"
 )
@@ -23,37 +22,17 @@ func BenchmarkRoundTrip(b *testing.B) {
 			return nil, nil
 		}),
 	}
-	ctxClient := &jrpc2.ClientOptions{EncodeContext: jctx.Encode}
 	tests := []struct {
 		desc string
 		cli  *jrpc2.ClientOptions
 		srv  *jrpc2.ServerOptions
 	}{
-		{"C01-CTX-B", nil, &jrpc2.ServerOptions{DisableBuiltin: true, Concurrency: 1}},
-		{"C01-CTX+B", nil, &jrpc2.ServerOptions{Concurrency: 1}},
-		{"C04-CTX-B", nil, &jrpc2.ServerOptions{DisableBuiltin: true, Concurrency: 4}},
-		{"C04-CTX+B", nil, &jrpc2.ServerOptions{Concurrency: 4}},
-		{"C12-CTX-B", nil, &jrpc2.ServerOptions{DisableBuiltin: true, Concurrency: 12}},
-		{"C12-CTX+B", nil, &jrpc2.ServerOptions{Concurrency: 12}},
-
-		{"C01+CTX-B", ctxClient,
-			&jrpc2.ServerOptions{DecodeContext: jctx.Decode, DisableBuiltin: true, Concurrency: 1},
-		},
-		{"C01+CTX+B", ctxClient,
-			&jrpc2.ServerOptions{DecodeContext: jctx.Decode, Concurrency: 1},
-		},
-		{"C04+CTX-B", ctxClient,
-			&jrpc2.ServerOptions{DecodeContext: jctx.Decode, DisableBuiltin: true, Concurrency: 4},
-		},
-		{"C04+CTX+B", ctxClient,
-			&jrpc2.ServerOptions{DecodeContext: jctx.Decode, Concurrency: 4},
-		},
-		{"C12+CTX-B", ctxClient,
-			&jrpc2.ServerOptions{DecodeContext: jctx.Decode, DisableBuiltin: true, Concurrency: 4},
-		},
-		{"C12+CTX+B", ctxClient,
-			&jrpc2.ServerOptions{DecodeContext: jctx.Decode, Concurrency: 12},
-		},
+		{"C01-B", nil, &jrpc2.ServerOptions{DisableBuiltin: true, Concurrency: 1}},
+		{"C01+B", nil, &jrpc2.ServerOptions{Concurrency: 1}},
+		{"C04-B", nil, &jrpc2.ServerOptions{DisableBuiltin: true, Concurrency: 4}},
+		{"C04+B", nil, &jrpc2.ServerOptions{Concurrency: 4}},
+		{"C12-B", nil, &jrpc2.ServerOptions{DisableBuiltin: true, Concurrency: 12}},
+		{"C12+B", nil, &jrpc2.ServerOptions{Concurrency: 12}},
 	}
 	for _, test := range tests {
 		b.Run(test.desc, func(b *testing.B) {

client.go

@@ -20,7 +20,6 @@ type Client struct {
 	done *sync.WaitGroup // done when the reader is finished at shutdown time
 
 	log   func(string, ...interface{}) // write debug logs here
-	enctx encoder
 	snote func(*jmessage)
 	scall func(context.Context, *jmessage) []byte
 	chook func(*Client, *Response)
@@ -41,7 +40,6 @@ func NewClient(ch channel.Channel, opts *ClientOptions) *Client {
 	c := &Client{
 		done:  new(sync.WaitGroup),
 		log:   opts.logFunc(),
-		enctx: opts.encodeContext(),
 		snote: opts.handleNotification(),
 		scall: opts.handleCallback(),
 		chook: opts.handleCancel(),
@@ -421,7 +419,7 @@ func (c *Client) stop(err error) {
 // value of params must be either nil or encodable as a JSON object or array.
 func (c *Client) marshalParams(ctx context.Context, method string, params interface{}) (json.RawMessage, error) {
 	if params == nil {
-		return c.enctx(ctx, method, nil) // no parameters, that is OK
+		return nil, nil // no parameters, that is OK
 	}
 	pbits, err := json.Marshal(params)
 	if err != nil {
@@ -432,11 +430,7 @@ func (c *Client) marshalParams(ctx context.Context, method string, params interf
 		// an array or an object.
 		return nil, &Error{Code: code.InvalidRequest, Message: "invalid parameters: array or object required"}
 	}
-	bits, err := c.enctx(ctx, method, pbits)
-	if err != nil {
-		return nil, err
-	}
-	return bits, err
+	return pbits, nil
 }
 
 func newPending(ctx context.Context, id string) (context.Context, *Response) {