Column Sanity

I think this PR is finally ready to be merged (+/- some things, I'm sure). It's basically an entire rewrite of the `Column` type and everything that entails. The focus for this rewrite was on speed and always forcing columns back to strict/dense version whenever possible.
Ah - will add that to the docs. Unfortunately, we can't use `n / 64` and `n % 64`, since both of those don't behave correctly when `n < 0`. We are treating `n` as an unsigned `Int` here, so need to use the shift without carry (`>>> 64`) and a mod that always returns a non-negative numbers (`& 0x3F`)