implement constraints #176
Description
I know this is a planned feature (listed in the README), but I saw there is no issue corresponding to this yet, so decided to create one!
It'd be nice to be able to express explicit constraints such as size(vec1) == size(vec2). Or size(vec1) == element. This constraint could then be checked with a "validate" function, i.e. not during "normal" reading, as that would slow things down.