More functions for working with SomeIntegers

[konsumlamm]

Currently, there are inc, dec, succ, pred for adding/subtracting an int. Internally, there are also functions for comparing with int32.

We should add more functions for working with BigInts and SomeIntegers, to avoid having to construct BigInts (and thus allocating) for small numbers. This would make sense for at least ==, <, +, -, *, div (in the second argument), mod (in the second argument).