provide a mode to give 8-byte alignment

[kuszmaul]

Linux libc provides 8-byte-aligned data.

It would be good if there was a mode in which supermalloc provided 8-byte aligned data. Right now we have sizes such as 10, 12, 14, 20, and 28, which are not aligned.

Another useful mode would be if the object is 16 bytes or bigger to make it 16-byte aligned (and also 32).

Another useful mode would be to make sure that values are not 16-byte aligned (for debugging codes that might use libc malloc in the future. I'm not sure this is a good idea, however.