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.

Co-authored-by: Graham Christensen <graham@grahamc.com>

src/libutil-tests/checked-arithmetic.cc

@@ -21,7 +21,7 @@ struct Arbitrary<nix::checked::Checked<T>>
     }
 };
 
-}
+} // namespace rc
 
 namespace nix::checked {
 
@@ -155,4 +155,4 @@ TEST(Checked, div_signed_special_cases)
     checkDivision<int16_t, int64_t>(0, 0);
 }
 
-}
+} // namespace nix::checked