gf-core/transfer

Some features of the Transfer language:

* Purely functional
* Dependent types
* Eager evaluation
* Generalized algebraic datatypes
* Metavariables
* Records with subtyping
* Overloading by explicit dictionary passing
* Pattern matching by case expressions

Additional features in the front-end language:

* Disjunctive patterns
* do-notation
* Hidden arguments (not implemented yet)
* Automatic derivation of some operations on user-defined GADTs:
  - Compositional maps and folds
  - Equality
  - Ordering
  - Showing
* Pattern equations
* Operator syntax for common functions, most are overloaded