Implement memalign

[tomrittervg]

The memalign functions are the only (I believe) functions missing from PartitionAlloc that would allow this to be used as a memory replacement library in Firefox. Which would be cool. =) https://glandium.org/blog/?p=2848

I'm hopeful that future versions of Firefox, once jemalloc with partitioning support gets upstreamed, update the replacement API to include partition requests, which would really enable a full replacement.

[struct]

I will look into this. Added to my priority 'todo' list.