Set-theoretic types: type definitions #13881

This refers to second and third milestones of Elixir's type system.

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
  • Type-checking of for-comprehensions
• Define a construct for Erlang FFI
• Define a construct for typing configuration

Implementation:

• Strong arrows
• Exhaustive type checking (case, try, receive, with, etc)

Additional theoretical concepts:

• Row polymorphism
• Behaviours
• Witness generation / property based testing