buffer: faster type check
Also add support for any TypedArray as target.
buffers/buffer-copy.js n=6000000 partial='true' bytes=128 *** 50.28 % ±4.26% ±5.68% ±7.42%
buffers/buffer-copy.js n=6000000 partial='true' bytes=8 *** 45.58 % ±4.53% ±6.04% ±7.89%