Expose length of buffered WebSocket data

Some encodings don't know how much data they need, rather they must
probe the data stream until they find an end marker. Expose how much
data is buffered in order to make this search efficient.

core/websock.js

@@ -124,6 +124,10 @@ export default class Websock {
         return res >>> 0;
     }
 
+    rQlen() {
+        return this._rQlen - this._rQi;
+    }
+
     rQshiftStr(len) {
         let str = "";
         // Handle large arrays in steps to avoid long strings on the stack

tests/test.websock.js

@@ -47,6 +47,20 @@ describe('Websock', function () {
             });
         });
 
+        describe('rQlen())', function () {
+            it('should return the number of buffered bytes in the receive queue', function () {
+                websock._receiveData(new Uint8Array([0xab, 0xcd, 0x12, 0x34,
+                                                     0x88, 0xee, 0x11, 0x33]));
+                expect(sock.rQlen()).to.equal(8);
+                sock.rQshift8();
+                expect(sock.rQlen()).to.equal(7);
+                sock.rQshift16();
+                expect(sock.rQlen()).to.equal(5);
+                sock.rQshift32();
+                expect(sock.rQlen()).to.equal(1);
+            });
+        });
+
         describe('rQshiftStr', function () {
             it('should shift the given number of bytes off of the receive queue and return a string', function () {
                 websock._receiveData(new Uint8Array([0xab, 0xcd, 0x12, 0x34,