Add 'instance UniformRange Natural'

[curiousleo]

The main addition here is to add instance UniformRange Natural, also see haskell#44.

To do this, I changed the Integer-specific uniformIntegerM to a generic uniformIntegralM. Since this function was and is recursive, I changed the pragma from INLINE to INLINEABLE.

I made sure that these more generic functions are no slower than before. Note that random/Integer benchmarks small ranges, for which a fast path is triggered; uniformR/unbounded/Integer and the new uniformR/unbounded/Natural benchmark very large ranges where there is no fast path.

Before (9ee79a7):

pure/random/Integer                      mean 362.6 μs  ( +- 44.38 μs  )
pure/uniformR/unbounded/Integer          mean 189.3 ms  ( +- 8.531 ms  )

After (this PR):

pure/random/Integer                      mean 369.0 μs  ( +- 36.89 μs  )
pure/uniformR/unbounded/Integer          mean 122.8 ms  ( +- 6.044 ms  )
pure/uniformR/unbounded/Natural          mean 110.6 ms  ( +- 16.08 ms  )

Other changes:

• specs use uniformR instead of randomR; this allow us to make the constraints more precise
• added Natural benchmark and specs
• sorted some imports alphabetically

[idontgetoutmuch]

Nice

[lehins]

LGTM