Disambiguate precedence (biased 'or')

[noughtmare]

We should introduce a biased <<|> operator which only backtracks if its left argument fails but otherwise commits to the first branch. This can be used to disambiguate precedence, e.g.

       (+) <$> expr <* char '+' <*> expr 
  <<|> (*) <$> expr <* char '*' <*> expr