Fix printing of (local_ x) |> f and some related forms

[ccasin]

This fixes an issue where:

let _ = (local_ x) |> f

was being printed as

let _ = local_ x |> f

which is a different thing, triggering the round trip check. I fixed this by adding a parenthesization rule for the lhs of infix operators - an alternative would be to add logic in the printing of infix operators, but since that is split out into several cases this felt cleaner.

This also fixes (and adds tests for) exclave_ in the same position. That is nonsense, but ocamlformat shouldn't blow up on it.