buffer: optimize buffer.concat performance

[mertcanaltin]

Removed the _copyActual indirection in the copy loop and called TypedArrayPrototypeSet directly.
Split auto-length and explicit-length paths so the auto-length copy loop is branch free. Replaced Buffer.allocUnsafe with allocate to skip redundant validation.

benchmark results:

➜  node git:(mert/buffer-concat-optimize-js) ✗ node-benchmark-compare ./result.csv
                                                                            confidence improvement accuracy (*)    (**)   (***)
buffers/buffer-concat-fill.js n=800000 extraSize=1                                   *     10.37 %       ±9.47% ±13.37% ±19.13%
buffers/buffer-concat-fill.js n=800000 extraSize=1024                                       2.91 %       ±6.22%  ±8.53% ±11.62%
buffers/buffer-concat-fill.js n=800000 extraSize=256                                 *      7.05 %       ±6.13%  ±8.41% ±11.49%
buffers/buffer-concat.js n=800000 withTotalLength=0 pieceSize=1 pieces=16           **      1.76 %       ±1.18%  ±1.64%  ±2.26%
buffers/buffer-concat.js n=800000 withTotalLength=0 pieceSize=1 pieces=4           ***      5.20 %       ±1.21%  ±1.65%  ±2.25%
buffers/buffer-concat.js n=800000 withTotalLength=0 pieceSize=16 pieces=16          **      1.96 %       ±1.17%  ±1.61%  ±2.21%
buffers/buffer-concat.js n=800000 withTotalLength=0 pieceSize=16 pieces=4          ***      3.06 %       ±1.50%  ±2.06%  ±2.84%
buffers/buffer-concat.js n=800000 withTotalLength=0 pieceSize=256 pieces=16                 1.93 %       ±8.35% ±11.48% ±15.73%
buffers/buffer-concat.js n=800000 withTotalLength=0 pieceSize=256 pieces=4                  2.37 %       ±6.69%  ±9.34% ±13.12%
buffers/buffer-concat.js n=800000 withTotalLength=1 pieceSize=1 pieces=16                  -0.59 %       ±0.99%  ±1.36%  ±1.86%
buffers/buffer-concat.js n=800000 withTotalLength=1 pieceSize=1 pieces=4           ***      5.17 %       ±1.25%  ±1.71%  ±2.33%
buffers/buffer-concat.js n=800000 withTotalLength=1 pieceSize=16 pieces=16                  0.98 %       ±1.45%  ±2.02%  ±2.81%
buffers/buffer-concat.js n=800000 withTotalLength=1 pieceSize=16 pieces=4          ***      4.44 %       ±1.89%  ±2.62%  ±3.62%
buffers/buffer-concat.js n=800000 withTotalLength=1 pieceSize=256 pieces=16          *     -1.97 %       ±1.89%  ±2.63%  ±3.67%
buffers/buffer-concat.js n=800000 withTotalLength=1 pieceSize=256 pieces=4         ***      3.55 %       ±1.08%  ±1.49%  ±2.04%

Be aware that when doing many comparisons the risk of a false-positive result increases.
In this case, there are 15 comparisons, you can thus expect the following amount of false-positive results:
  0.75 false positives, when considering a   5% risk acceptance (*, **, ***),
  0.15 false positives, when considering a   1% risk acceptance (**, ***),
  0.01 false positives, when considering a 0.1% risk acceptance (***)
➜  node git:(mert/buffer-concat-optimize-js) ✗