Don't allow improper_ctypes

Most of what this crate does is interact with C, so the lint should be
useful. It can be disabled on a case-by-case basis as needed.

There are a few fixes needed for enums, which will be going away anyway
(#4419).

Additionally, switch the `bad_style` lint to the newer name
`nonstandard_style`.

(backport <#4584>)
(cherry picked from commit 38bb46e)

Author: tgross35

src/lib.rs

@@ -4,9 +4,8 @@
 #![allow(
     renamed_and_removed_lints, // Keep this order.
     unknown_lints, // Keep this order.
-    bad_style,
+    nonstandard_style,
     overflowing_literals,
-    improper_ctypes,
     unused_macros,
     unused_macro_rules,
 )]