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 #14558 and can only be started once they are completed.

New constructs and APIs will be added to Elixir:

• [ ] Define a construct for typed structs (field documentation, parametric definitions, private fields)
• [ ] 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
  • [ ] Dispatching based on output type
• [ ] 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
  • [ ] Anonymous function call inlining
  • [ ] Augment type inference to also consider parametric types
  • [ ] Recursive types in for-comprehensions' reduce
• [ ] Row polymorphism
• [ ] Behaviours
• [ ] Witness generation / property based testing