Add a section on subtyping

[beta-ziliani]

To start with, given that probably the semantics are still a bit in flux, it can be a stub with just the basics.

[HertzDevil]

I worked on this a bit a while ago, but never got around submitting a PR because I'm not even sure if the docs are the most appropriate place for them. The rules are:

• Idempotency: every type is a subtype of itself.
• Transitivity: if T is a subtype of U and U is a subtype of V, then T is also a subtype of V.
• A class or struct type is a subtype of its superclass.
• A type is a subtype of the modules it includes.
• A class type is a subtype of the modules its instance type extends.
• A generic instance type is a subtype of its corresponding uninstantiated generic type.
• If T and U are instances of Tuple, then T is a subtype of U if the two types have the same number of elements and each element type in T is a subtype of the corresponding element type in U.
• If T and U are instances of NamedTuple, then T is a subtype of U if the two types have the same keys and each value type in T is a subtype of the corresponding value type in U.
• A type T is a subtype of a union if T is a subtype of at least one variant type of that union.
• A union type is a subtype of type T if all variant types of that union are subtypes of T.
• An alias type is a subtype of the type it refers to, and vice-versa.
• If T is a typedef of U, then T and U are sibling types; T is a subtype of U's supertypes, but neither T nor U is a subtype of one other.
• T.class is a subtype of U.class if T is a subclass of U, or if T is a generic instance of U. (Subtyping rules for metaclasses are pending a decision for Ancestors of module metaclasses are inconsistent crystal#11110.)
• NoReturn is a subtype of any type.

There would be examples for each of those rules using TypeNode#<=.

[beta-ziliani]

If not in the book, where? I mean, some examples could go into a tutorial, but I wouldn't expect it to cover all these rules, just the most used ones.

[straight-shoota]

I think the language reference is exactly where this needs to be documented.

[revskill10]

I'm confused a bit with subclass is a subtype of class.

Actually subclassing and subtyping are orthogonal concepts ?