Allow side conditions in function clauses and lambda clauses

The PR

* #2852 

adds side condition syntax only for case-expressions. For the sake of syntax uniformity, we should allow them anywhere pattern matching is allowed, e.g.,
```
f (lst : List Nat) : Nat
  | [x]
       | if F x := A
       | if G x := B
       | else := C
  | _ := D;
```
