Reducing the amount of information read from the table file for variant types

[amblafont]

The sum constructor of uval (in ML) only needs a list of booleans (indicating whether each constructor is checked) rather than the full list of the types of the constructors.

I trust the ML typechecker, so I don't expect any bad outcome of testing bilby with this change. I would let @vjackson725 perform the test, as the README file for bilby scares me :)