Ensure ES2016 engines construct Uint8Array (not Buffer) from Buffer.prototype.slice

Ensure ES2016 engines construct Uint8Array (not Buffer) from Buffer.prototype.slice

In the ES2016 draft specification, TypedArray methods like
%TypedArray%.prototype.subarray() call out to a constructor for the result
based on the receiver. Ordinarily, the constructor is instance.constructor,
but subclasses can override this using the Symbol.species property on the
constructor.

Buffer.prototype.slice calls out to %TypedArray%.prototype.subarray, which
calls this calculated constructor with three arguments. The argument pattern
doesn't correspond to a constructor for Buffer, so without setting
Symbol.species appropriately, the wrong kind of result is created.

This patch sets Buffer[Symbol.species] to Uint8Array when appropriate, to
address the issue.