feat: teach VarBinArray to only compute IsConstant when requested

[danking]

I also remove the is_strict_sorted check because it provides litle value and is rather expensive, particularly now that we need not compute all statistics to know if a VarBinArray is constant.

[danking]

I'm not certain this is any faster, so let's look at the benchmarks first.

[robert3005]

yeah, I see itertools has annoying bugs regarding sized, i.e. you can't use them on dyn traits

[robert3005]

@danking could be the case that the varbiniterator is the slow thing. It's very naive the way it's written and we likely need a better version of it

[danking]

Clearly some benefit to compression time but decompress is impaired in a few cases. We don’t have enough samples yet to know how much variance there is.

[danking]

This is not clearly better than develop.