Fixed finToBits using the Idris implementation of nextPow2

Data.Bits already has a nice next power of 2 function. However,
it is not exported. It should be useful for the DEA library anyway
so we import it. This fixes the problem with just using log2 for the
finToBits signature: i.e. we want a ceiling-oriented log2 function,
which is exactly what nextPow2 is.
Merge pull request #2 from esmooov/fix/master/finToBits

Fixed finToBits using the Idris implementation of nextPow2
Fixed natToMod to successfully typecheck

Specified that unless both the Nat is (S n) and the target type is Mod (S m),
natToMod should return mZ