Add (U)?Int(128|256) integer types.

[palkeo]

I had to implement read_items/write_items given that this size of integers is not supported by Python's struct.

Not sure it's ideal to have these new types transitively inherit from FormatColumn, but I wanted to keep the check_items/before_write_items, and this works fine.

[long2ice]

Thanks! Please add unittests about this.

[palkeo]

Done.

[randomowo]

@long2ice when it will be merged?

[long2ice]

Thanks! Looks like ci was failed

[long2ice]

Thanks! Please rebase dev and make ci pass

[randomowo]

i thing using asserts in Big(U)IntColumn is a bad idea if only because if you use python with -O flag asserts would not work.

[randomowo]

@palkeo @long2ice is that okey if i reopen this (and #45) pr from my fork for faster merge?

[long2ice]

@randomowo That's good

[randomowo]

#57