BenchmarkResults fields should almost certainly use lenses

[rrnewton]

As Issue #73 highlights, we really want to be able to refer to the "columns" (fields within the BenchmarkResult record) as first class entities.

This will be a pretty wide ranging change and will also introduce a substantial new dependency. Best to save for 2.0.

[rrnewton]

There may still be an issue with (de)serializing the lenses.

At least in the DefaultParamMeaning context we need to be able to say WHICH field we're operating on, and that context (arbitrary Haskell code) it may be fine to just include the first class Lens' or Setter directly.

But there may be other contexts where we need to describe which field we're operating on as a String, so there will probably still be a boring dispatch function that needs to be maintained.