[whatwg/encoding] TextEncoder#encode - write to existing Uint8Array (#69)

For binary protocols, such as msgpack, performance of writing/reading strings can be crucial, however those strings rarely (virtually never) exist on a binary buffer on their own. When decoding a string from a binary buffer of Uint8Array (obtained over, say, websockets) it's possible to cheaply create a subarray and pass it over to `TextDecoder#decode`, as such the decoding story is pretty straight forward.

The writing story is much more complicated.

In order to write a string into an existing buffer, one must take the result `Uint8Array` from `TextEncoder#encode` and `TypedArray#set` it at a desired offset into another buffer, which introduces unnecessary copying and, as result, is slower than plain JavaScript encoding implementations.

A much better solution would be analog to the Node.js [`Buffer#write` implementation](https://nodejs.org/api/buffer.html#buffer_buf_write_string_offset_length_encoding) where the encoder can write a string to a buffer at a desired offset. Example signature could be something like:

```
TextEncoder#write(DOMString source, Uint8Array target [, Number offset [, Number length]])
```

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