perf(stats): implement SIMD accelerated statistics + misc refactor

- Free functions like `mean`, `std_dev` has been removed, use the equivalent
  associated methods in the new `Stats` trait
- The `slope` method has been removed from `StraightLine`, instead use the
  tuple indexing notation: `line.0`
- The vector construction part of the tests has been DRYed.

[breaking-change]
Merge pull request #11 from japaric/simd

perf(stats): implement SIMD accelerated statistics + misc refactor