Speed up buffer creation.

Speed up buffer creation.

Since 1d20f50f8b4c62779c0dbfb1fa401168ab52dce0 (#40), instances already
gained all the `Buffer`'s methods via the prototype re-assignment.
Skipping the `_augment` call yields significant speedups:

~50% faster small buffer creation:

  new(16) before: 595,757 ops/sec ±0.92% (91 runs sampled)
  new(16) after: 894,462 ops/sec ±0.69% (87 runs sampled)

~20% faster slice:

  slice before: 2,176,194 ops/sec ±1.59% (89 runs sampled)
  slice after: 2,624,209 ops/sec ±0.42% (99 runs sampled)

Other benchmarks aren't significantly different.