Generate signatures for field implementations in instances

[valis]

Signature of functions defined in instances (and other functions) must be specified explicitly. It is possible to generate them automatically. This can be implemented as an intention.

Before:

\class D
  | func (x y : Nat) : Nat

\instance D-inst : D
  | func => {?}

After:

\instance D-inst : D
  | \infix 3 func (x y : Nat) : Nat => {?}

[sxhya]

Is it ok that the example in "after" does not typecheck correctly (it reports the error "elim expected got =>")