Allow no-op type coercions within @fold scopes.

[obi1kenobi]

Using @fold on vertex fields of union type was practically impossible, since the union type requires a type coercion, and type coercions are disallowed within @fold scopes. This PR extends the use of the type_equivalence_hints optional compilation parameter, allowing the compiler to detect "no-op" coercions (e.g. from the union type to the type that a superclass of all other types in the union) and allow them inside @fold scopes.

See the updated test cases for examples.

[coveralls]

Coverage decreased (-0.02%) to 93.757% when pulling 9301cb4 on coerce_to_base_type_within_fold into e8d4249 on master.

[coveralls]

Coverage decreased (-0.02%) to 93.757% when pulling 7c9f87a on coerce_to_base_type_within_fold into e8d4249 on master.

[coveralls]

Coverage increased (+0.03%) to 93.805% when pulling 38221e5 on coerce_to_base_type_within_fold into e8d4249 on master.