Add `async_` and `const_` field in `FnSig`

[zjp-CN]

FnSig currently is a reflection of internal FnSig, so no function modifier information like asyncness or constness.

To obtain the asyncness, TyCtxt::asyncness is available. (Same for constness.)

However, adding extra fields will make internal<->stable bridge more fragile, like RustcInternal impl on FnSig needs to trim these fields, and Stable impl on FnSig needs to query more to piece them together.