Fixed type lookup: types were looked up from "self", but they should be looked up from the matching def.

When using a union as a restriction, bind free variables  from left to right whiel discarding matching types

Removed CastFunToReturnVoid AST node and use a regular Cast node instead. Allow casting a function to another one that returns void.

Disable passing a block as a function. Disable accidental creation of closures (which lead to a compiler crash) until we implement closures. Error when using "yield" or "return" in a function literal.

In codegen, when visiting a yield there must be a block context.