Compilation flag to build without singletons

[int-index]

I use this library to prove KnownNat (n + 1) from KnownNat n, installing singletons is a bit of an overkill.

[christiaanb]

It’s never really bothered me personally I guess, because a lot of my code depends on singletons, so I used some of its types without too much thought.

Removing the dependency shouldn’t be too much work I think.

[christiaanb]

I've removed the singletons dependency and will probably do a release between Christmas and New Years.

[int-index]

Thanks!