PUP-3366 Fix issues with string/enum type assignable calculations

There were errors in the calculation of string/enum calculation.
Basically Enum == String, if String is size constrained then
Enum < String. This fixes this calculation.

There were no tests for this, they are now added.
(PUP-3366) Correct comparisson of Enum > Enum[a]

Comparissons involving an unparentesized Enum were not correct
because an iteration with all? returns true if the set being
enumerated is empty.

This modifies the logic, and adds missing tests.
Merge pull request #3134 from hlindberg/PUP-3366_string-enum-type-calculation

PUP-3366 Fix issues with string/enum type assignable calculations