Support linear regression in a general way

[rrnewton]

As of Criterion 1.0, it uses a linear regression methodology to generate an estimate of the expected marginal cost of running a benchmark just one more time.

A core type in Criterion is the Benchmarkable data structure, which has an exposed constructor and is of the form:

data Benchmarkable = Benchmarkable (Int64 -> IO ()) 

Usually, the user doesn't construct one of these objects directly, rather criterion constructs the Int64 -> IO () function that simply takes a number and runs an IO action N times. However, there are useful applications of constructing this function directly:

• Ruling out overhead from some startup or initialization action. The regression methodology will do that automatically and it's ok to have actions of the form \n -> do init; realStuff n.
• Running code inside a different (non-IO) monad, like the Par monad. Doing a runPar sets up an execution environment. This is an example of a one-time cost that can be amortized over a loop that runs /inside/ the Par monad.
• Varying something other than the number of iterations. For example, it is also useful to do regressions while varying data structure size rather than number of iterations.

The reason I'm mentioning this here, is that HSBencher/Criterion integration would make sense, and could take two distinct forms:

• We should make it possible to parameterize arbitrary HSBencher benchmarks by a numeric parameter, and perform linear regression. This works even though traditional HSBencher benchmarks run in their own process.
• Second, for Haskell code, we should make it possible to run traditional, intra-Haskell Criterion benchmarks and make it easy to dump their outputs as HSBencher-harvestable tags that go to our backend data stores.

In the second case, it's not clear to me whether the benchmarks should run in the same process as the benchmark harness or not. Opinions on that welcome.

[rrnewton]

CC @tmcdonell @svenssonjoel . Btw, one reason we want something like this is projects like ConcurrentCilk have some benchmarks where we want to do regression, but they don't use Haskell at all. (Except in the HSbencher script.)