Fix parenthesization for binary operators

[HarrisL2]

A small modification that shouldn't impact other code, fixes part of #65

[LPTK]

Thanks for the quick fix!

Only thing is that there's already shared/src/test/diff/codegen/Parentheses.mls, so better move your test cases there.

Also, let's add this one:

fun (--) minusminus(a, b) = a - b
//│ fun (--) minusminus: (Int, Int) -> Int

:js
1 -- (3 -- 5)
//│ Int
//│ // Prelude
//│ class TypingUnit3 {}
//│ const typing_unit3 = new TypingUnit3;
//│ // Query 1
//│ res = minusminus(1, minusminus(3, 5));
//│ // End of generated code
//│ res
//│     = 3

It doesn't have a problem because custom operators are desugared to plain methods. It was not clear to me that this is why your condition JSBinary.operators.contains(op) is sufficient in JSBackend.scala. Better add a comment to that effect there!

[LPTK]

The test is not clean. Please remove this and finally get this PR merged...

[LPTK]

Never mind, I did it myself.