feat: relaxes grammar to allow un-parenthesized unary operator patterns

[christoph-dfinity]

As there's no ambiguity arising from this, I think it's consistent and more convenient to group unary operator patterns (like case -1) with the other literals.

[github-actions]

Comparing from 82171b1 to c6bc1e0:
In terms of gas, no changes are observed in 5 tests.
In terms of size, no changes are observed in 5 tests.

[crusso]

I guess that's ok, but I think it might break the symmetry with expressions:

crusso@crusso-Virtual-Machine:~/motoko/src$ rlwrap moc
Motoko compiler (source 0.16.3-40-g5e9da46fa)
> func id(x:Int):Int{x};
let id : (x : Int) -> Int = <func>
> id -1;
stdin:2.5-2.6: type error [M0050], literal of type
  Nat
does not have expected type
  (x : Int) -> Int
> id (-1);
-1 : Int
> -1;
-1 : Int

I must admit, I have a hard time understanding this error:

> id -1;
stdin:2.5-2.6: type error [M0050], literal of type
  Nat
does not have expected type
  (x : Int) -> Int