Refactor CountStats state machine

[dbaggerman]

Since I've been back in the scc code, I got tempted to make an(other) attempt to break up the complexity in the CountStats state machine.

This changes from states being an enum, with all the state tracked in CountStats and passed in/out of the different methods to introduce a State interface with different states having their stateXYZ function moved to being attached to the relevant struct. This means we can drop the switch statement in the core loop and instead let the go runtime do the dispatch. It also lets relevant state be tracked in the State struct instead of being passed in and out of CountStats.

As a bonus, the different 'flavours' of slComment vs slCommentCode, etc are no longer required.

It seems to produce the same result in the examples I've thrown at it, other than python docstrings - issue #246 doesn't exist in this implementation (this mismatch is how I found that issue in the first place). It could probably use a bit more hammering just to make sure though - I haven't tested permutations of optional flags for example.

Performance is roughly on par with the current implementation, only being marginally slower:

Benchmark #1: ./scc-master ../../torvalds/linux
  Time (mean ± σ):     688.1 ms ±  11.9 ms    [User: 8.613 s, System: 0.970 s]
  Range (min … max):   663.2 ms … 712.3 ms    50 runs

Benchmark #2: ./scc-refactor ../../torvalds/linux
  Time (mean ± σ):     695.7 ms ±  10.4 ms    [User: 8.567 s, System: 1.034 s]
  Range (min … max):   675.1 ms … 720.2 ms    50 runs

Summary
  './scc-master ../../torvalds/linux' ran
    1.01 ± 0.02 times faster than './scc-refactor ../../torvalds/linux'

[boyter]

Yeah the state machine stuff is something I have been toying around with as well. I want too start pulling out common languages into their own versions which should allow for further performance optimisations. For example, counting text files should be very fast as they only need to check for new lines, and if the line is all whitespace to produce the correct counts.

Something ill get back into at some point.