Use Uint8Array instead of Buffer in Node; faster string writing

[mourner]

• Got rid of the buffer shim; now both Node and browsers use Uint8Array under the hood for consistency and simpler code (closing Replace buffer implementation with standard Uint8Array #35)
• Made string writing slightly faster — it now uses the same technique as writeMessage (reserving 1 byte for length and then shifting data if it's more than 1 instead of encoding into a temporary array of bytes)
• Fixed the array copying behavior when decoding in the browser
• Decoding benchmark performance dropped by ~11% in Node, but I think this is fine
• Encoding benchmark performance increased by ~14% in Node
• Browser benchmark results are the same

cc @jfirebaugh @kjvalencik

[jfirebaugh]

Excellent!

[kjvalencik]

IMHO, this is a pretty substantial change because anything that is expecting a Buffer may either break or convert to a buffer which is a copy operation.

In your experience, is copying large-ish chunks of memory like this a big performance cost?

[tmcw]

This'll be a 3.x, right, so dependents wouldn't automatically upgrade.

[mourner]

@kjvalencik it's a breaking change, so it prompts a semver-major release (along with some more subtle changes after v2.0.1). It's easy to adapt to though — you can do either Buffer.from(arr) (copy) or Buffer.from(arr.buffer) (shares memory) if you depend specifically on the Buffer API (which is already implemented internally as Uint8Array as of Node 4 AFAIR).