Flip operator precedence rules for pretty-printing

The operator precedence rules for pretty-printing appeared to be backwards from what we intended, so I flipped them. In the process, I tweaked some of the other parenthesization logic: so far, everything I've thrown at the pretty-printer seems satisfactory, but it is hard to exhaustively check the cases!

Fixes issues https://github.com/idris-lang/Idris-dev/issues/1559 and https://github.com/idris-lang/Idris-dev/issues/1650.