feat(parsers): ByteLength is now more efficient (#1402)

And better documented

lib/parsers/byte-length.js

@@ -5,14 +5,16 @@ const Transform = require('stream').Transform;
 /**
  * A transform stream that emits data as a buffer after a specific number of bytes are received.
  * @extends Transform
+ * @param {Object} options
+ * @param {Number} options.length the number of bytes on each data event
  * @example
-To use the `ByteLength` parser, provide the length of the number of bytes:
+To use the `ByteLength` parser:
 ```js
 const SerialPort = require('serialport');
 const ByteLength = SerialPort.parsers.ByteLength
 const port = new SerialPort('/dev/tty-usbserial1');
 const parser = port.pipe(new ByteLength({length: 8}));
-parser.on('data', console.log);
+parser.on('data', console.log); // will have 8 bytes per data event
 ```
  */
 class ByteLengthParser extends Transform {
@@ -29,23 +31,28 @@ class ByteLengthParser extends Transform {
     }
 
     this.length = options.length;
-    this.buffer = Buffer.alloc(0);
+    this.position = 0;
+    this.buffer = Buffer.alloc(this.length);
   }
 
   _transform(chunk, encoding, cb) {
-    let data = Buffer.concat([this.buffer, chunk]);
-    while (data.length >= this.length) {
-      const out = data.slice(0, this.length);
-      this.push(out);
-      data = data.slice(this.length);
+    let cursor = 0;
+    while (cursor < chunk.length) {
+      this.buffer[this.position] = chunk[cursor];
+      cursor++;
+      this.position++;
+      if (this.position === this.length) {
+        this.push(this.buffer);
+        this.buffer = Buffer.alloc(this.length);
+        this.position = 0;
+      }
     }
-    this.buffer = data;
     cb();
   }
 
   _flush(cb) {
-    this.push(this.buffer);
-    this.buffer = Buffer.alloc(0);
+    this.push(this.buffer.slice(0, this.position));
+    this.buffer = Buffer.alloc(this.length);
     cb();
   }
 };