Support more than 64 threads

[chriselrod]

See #151 (comment) for a brief introduction.

If someone wants to tackle this, I'd be happy to answer any questions.

You may also want to look at how Polyester handles it:
https://github.com/JuliaSIMD/Polyester.jl/blob/2926208439122399e4bd2f25727615a6d86e25c0/src/batch.jl#L142-L173
Except that apparently it isn't doing it correctly either
JuliaSIMD/Polyester.jl#83