clippy: allow `if_not_else`
This lint fires when an `if` negates the condition. The lint
instead suggests that the condition should be un-negated and
the `if` and `else` bodies flipped.
I find this to be a pretty annoying lint, because sometimes
I want to keep the order of the `if` and `else` bodies as-is,
and not make it subject to whether the primary condition is
negated or not. Sometimes it's for linear scanning reasons
(where the `if` block is smaller), and sometimes it's for
"code parallelism" reasons (i.e., this block of code looks like
another block of code intentionally).
Moreover, I don't think the benefits of this lint are very
big. I do agree that sometimes negating a conditional can make
it harder to read (e.g., a double negative), but it's nowhere
near a universal truth enough to ban it outright.
