Set-theoretic types: type definitions

[josevalim]

This refers to second and third milestones of Elixir's type system. It is a continuation of #13227 and can only be started once that is completed.

New constructs and APIs will be added to Elixir:

• Define a construct for typed structs (field documentation, parametric definitions, private fields, deprecate empty structs)
• Define a construct for type aliases
• Define a construct for type signatures
• Define a construct for typed behaviours
• Define a construct for typed protocols and implementations (parametric protocols)
• Define a construct for Erlang FFI

Implementation:

• Module-local inference of private functions based on callers
• Occurrence typing via reverse arrows
• Strong arrows

At the descr-level, we need to support:

• Recursive types
• Parametric polymorphism and anonymous function call inlining
• Row polymorphism
• Behaviours