Apply clang-format universally.

* It is tough to contribute to a project that doesn't use a formatter,
* It is extra hard to contribute to a project which has configured the formatter, but ignores it for some files
* Code formatting makes it harder to hide obscure / weird bugs by accident or on purpose,

Let's rip the bandaid off?

Note that PRs currently in flight should be able to be merged relatively easily by applying `clang-format` to their tip prior to merge.

src/libutil-tests/nix_api_util.cc

@@ -155,4 +155,4 @@ TEST_F(nix_api_util_context, nix_err_code)
     ASSERT_EQ(nix_err_code(ctx), NIX_ERR_UNKNOWN);
 }
 
-}
+} // namespace nixC