TASK: Reform of the `TypeInterface`

[grebaldi]

The TypeInterface currently looks like this:

interface TypeInterface
{
    public function is(TypeInterface $other): bool;
}

The method is is fairly ambiguous, since it could mean "equivalence" (= $other is actually the exact same type) or "implication" (= $other is a sub-type of $this).

So, I'd suggest to change the interface thusly:

interface TypeInterface
{
    public function equals(TypeInterface $other): bool;
    public function isSuperTypeOf(TypeInterface $other): bool;
}

An alternative name for isSuperTypeOf would be implies. The interface could also reverse the direction of query and have a method isSubTypeOf (or: isImpliedBy). Personally, I'm indifferent to either of these, but would strictly opt for having only one of those methods.

The rules for the built-in types in regards to isSuperTypeOf should be as follows:

Built-in Type	isSuperTypeOf($other) when...
BooleanType	equals($other)
ComponentType	equals($other)
EnumStaticType	equals($other)
EnumType	equals($other)
NumberType	equals($other)
SlotType	equals($other) or $other is one of: StringType, ComponentType
StringType	equals($other)
StructType	equals($other)
UnionType	equals($other) or $other is UnionType with fewer, but compatible members, or $other is any member type of this union