Speedy slice sampling.

This implementation of the slice sampling algorithm uses `lens`

as a means to
operate over generic indexed traversable functors, so you can expect it to
work if your target function takes a list, vector, map, sequence, etc. as its
argument.

Additionally you can sample over anything that's an instance of both `Num`

and
`Variate`

, which is useful in the case of discrete parameters.

Exports a `mcmc`

function that prints a trace to stdout, a `chain`

function
for collecting results in memory, and a `slice`

transition operator that can
be used more generally.

```
import Numeric.MCMC.Slice
import Data.Sequence (Seq, index, fromList)
bnn :: Seq Double -> Double
bnn xs = -0.5 * (x0 ^ 2 * x1 ^ 2 + x0 ^ 2 + x1 ^ 2 - 8 * x0 - 8 * x1) where
x0 = index xs 0
x1 = index xs 1
main :: IO ()
main = withSystemRandom . asGenIO $ mcmc 10000 1 (fromList [0, 0]) bnn
```