net/http: add ErrUseLastResponse, Client.CloseIdleConnections, Transport.Proxy and ProxyURL (#70)

* http: add ErrUseLastResponse and Client.CloseIdleConnections

Both are API-compatibility fills. ErrUseLastResponse is a sentinel a
CheckRedirect func returns to stop following redirects; nothing here has
to act on it beyond existing. CloseIdleConnections delegates to the
Transport when it has the method, which is what net/http does.

Programs that reference either currently fail to compile against this
package for want of a name, which is the whole cost.

http/transport.go

@@ -34,6 +34,12 @@ type readTrackingBody struct {
 // golang.org/x/net/http2 and google.golang.org/grpc. They are stored but, aside
 // from fetch, have no effect at runtime.
 type Transport struct {
+	// Proxy specifies a function to return a proxy for a given Request.
+	//
+	// TINYGO: present for API compatibility; the netdev-backed transport dials
+	// directly and does not consult it.
+	Proxy func(*Request) (*url.URL, error)
+
 	// TLSClientConfig specifies the TLS configuration to use with tls.Client.
 	TLSClientConfig *tls.Config
 
@@ -130,6 +136,14 @@ func ProxyFromEnvironment(req *Request) (*url.URL, error) {
 	return nil, nil
 }
 
+// ProxyURL returns a proxy function (for use in a Transport) that always returns
+// the same URL.
+func ProxyURL(fixedURL *url.URL) func(*Request) (*url.URL, error) {
+	return func(*Request) (*url.URL, error) {
+		return fixedURL, nil
+	}
+}
+
 // ErrSkipAltProtocol is a sentinel error value defined by Transport.RegisterProtocol.
 var ErrSkipAltProtocol = errors.New("net/http: skip alternate protocol")