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

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

Finally fixed the problem with assignment in the right hand side of an "and" making the variable nilable afterwards and, when in an "if"'s condition, not being nil.

Fixed #67: instances that never get instantiated get their fields typed as int8. Calls that are never instantiated raise "untyped exception", but that should never happen in safe code. On the other hand, breaking from a block unconditionally doesn't work right now (but that's not very useful).