Force constraint system completer to always fix variable with unknown position.
This makes inferred type for in parameters less specific, but OI infers to
subtype in such cases, so NI won't infer to less specific types than before.
Current selection of completion mode for call is not always correct in case of full mode,
and sometimes too conservative in case of partial mode. Updated algorithm checks constraints
wrt position of type variables in return type and in other related constraints.
Full completion happens if proper constraint requirements are satisfied for variables.