Allow chaining of varint method

Each method in Put, except for `varint` (which was apparently added later), supports chaining by returning a reference to `this`.

This pull request adds support for method chaining when using the `varint` method. For example, the following is now possible:

```
var buf = new Put()
   .word16be(ASSETS_VERSION)
   .varint(this.assetQuantities.length)
   .buffer();
```