Start working on commands

[kubouch]

This PR adds basic parsing, name binding and typechecking of def and signature parameters.

Closures and commands share the same parameter parsing code. Hopefully, we can grow it this way to enable more comprehensive typing of closures (e.g., input/output types).

Got sidetracked by improving the typechecker:

• complex types like list<list<int>> are supported in command signatures
• list type must have only one type param (list<int, string> and list<> are not allowed)
  • this is enforced at the typechecker, but we might want to push it all the way to the parser

It's not possible to call the defined commands yet, that will be a next PR.