Macro.to_string: don't add unneeded binop parens

This changes the `Macro.to_string` output of (quoted) `1+2+3+4` from
`((1 + 2) + 3) + 4` to `1 + 2 + 3 + 4`.
Merge pull request #1606 from pminten/macro-to-string-binop-parens

Macro.to_string: don't add unneeded binop parens