Rollback bindgen to 0.31 release

The 0.42 release of bindgen was failing in the docker rust image. All
versions between 0.31 and 0.42 failed with other errors, so this is the only
version that works on both macOS and docker.

cargo test passes

CHANGELOG.md

@@ -7,7 +7,7 @@ This file follows the convention described at
 
 ## [Unreleased]
 ### Changed
-- Updated `bindgen` dependency to latest release and fixed compiler issues.
+- Updated `bindgen` dependency to 0.31 release and fixed compiler issues.
   Enum definitions changed again, default in bindgen is different now, and
   using `default_enum_style()` caused endless compiler errors.
 

Cargo.toml

@@ -14,5 +14,5 @@ build = "build.rs"
 libc = ">=0.2"
 
 [build-dependencies]
-bindgen = "0.42"
+bindgen = "0.31"
 pkg-config = "0.3"