Fix ty() for reference

[chenyan-dfinity]

The current design of Func and Service cannot produce correct implementation of ty(), because you cannot decide the type for Func::ty(). ty() for references is only defined at the value level, not at the type level.

Two possible solutions:

1. Use closure and trait to represent function and service references
2. Special case on ty() when it's a reference. But it breaks compositionally...

[lastmjs]

Any ETA on this? I'm about to embark on implementing this generally in Azle and wondering if this might come out soon.