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/executable-path.cc

@@ -61,4 +61,4 @@ TEST(ExecutablePath, elementyElemNormalize)
     EXPECT_EQ(s2, OS_STR("." PATH_VAR_SEP "." PATH_VAR_SEP "." PATH_VAR_SEP "."));
 }
 
-}
+} // namespace nix