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.

Cargo.toml

@@ -222,6 +222,7 @@ get_unwrap = "warn"
 rc_buffer = "warn"
 rc_mutex = "warn"
 rest_pat_in_fully_bound_structs = "warn"
+if_not_else = "allow"
 
 [profile.release]
 strip = true