`define`d constructors should reside in the AST module

[andreasabel]

defined constructors should reside in the AST module, rather than just in the parser.

• Modify the TypeChecker to store the inferred types of the definition arguments.
• Place the definitions in the AST module (together with their type signatures).
• Java: cannot place into package Absyn, place in class AbsynDef

Backends:

• Agda
• Haskell
• C
• C++
• Java
• Ocaml