Enforce whitespace around binary operators

[jiribenes]

Resolves #462

Caution

Please don't merge this yet, I'm not happy about the implementation. Thanks!

The parser soft fails (fails with recovery) if it encounters a binary operator that is not surrounded by some whitespace:

// OK
val _ = 1 + 2
val _ = 1  + 2
val _ = 1 +  2
val _ = 1 +
  2

// FAIL
val _ = 1 +2
val _ = 1+ 2
val _ = 1+2

The implementation is a bit hacky, see the note in the comment.