Allow to use bitmaps with arbitrary size

[alovak]

In one of our cases, I found that we need a bitmap with 4 bytes. The current implementation of the field.Bitmap assumes that 8 bytes is the minimal bitmap size. We have to make it default, but allow us to specify a smaller size if needed.

There will probably be no change to API, as we already specify Length in the field.Spec, we just have to use it instead of hardcoded minBitmapLenght.