buffers: ensure alignment

[bnoordhuis]

Make sure that buffers (fast and slow both) are always aligned on 8 or 16 byte boundaries.

Unaligned loads and stores are unsafe/unpredictable/inefficient on the architectures we support. This is particularly relevant now that it's possible to create typed arrays (int16, int32, float, double) from buffers.

[bnoordhuis]

Partially fixed in 285d8c6.

[trevnorris]

@bnoordhuis this still an issue? If it is, should be able to fully implement it in the new Buffers.

[bnoordhuis]

No, it's been fixed.