yang-model-api: fix {Length,Range}Constraint

model.api.type.LengthConstraint does not correctly model YANG's length keyword, as it provides only upper and lower bound, effectively forcing

TypeDefinition types to report them as a List. This list is then interpreted as a set of ranges and the value needs to fit into at least one of them. Each of the ranges contains the same metadata, which makes error reporting a bit quirky.

The same problem exists with RangeConstraint.

Remodel these interfaces to include a RangeSet covering allowed ranges of values, so the type definitions only return a Optional<LengthConstraint>, against which validation is done.