Some of the arithmetic promoted all `Unsigned` to `Culong`. In the ziggurat code I had a `BigFloat/Uint64` where the `Uint64` was too big for Culong on 32 bit. Therefore I wanted to change the header from `Signed` to `SmallSigned`. However, given @tkelman's reporting I'd better define the alias `NoLargerThanCulongOnAnySystem` and leave `int.jl` untouched. I'll add some tests.