Disambiguate fixity

[noughtmare]

Currently, when you write something like Plus <$> expr <* char '+' <*> expr <|> number and use that on the string 1+2+3 you'll get two results: Plus 1 (Plus 2 3) and Plus (Plus 1 2) 3.

We lack a way to specify which one of these should be chosen. Perhaps a notation like Plus <$> expr <* char '+' <<*> expr <|> number could force it to only return Plus (Plus 1 2) 3.

I don't have a clear idea of how to accomplish this in the algorithm.