Changes

Summary

1. merged
2. simplified -- avoid odd mutable state, which potentially causes problems with module initialization;
3. back to static Mixfix.default_constraint without any special tricks (reverting e6443edaebff);
4. Type_Infer.object_logic controls improvement of type inference result;
5. tuned;
6. simplified constraints;
7. back to dummy constraints (amending dd2914250ca7): important for Syntax_Phases.get_free/is_declared;
8. tuned imports;
9. tuned message;
10. tuned;