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/libexpr-tests/nix_api_external.cc

@@ -27,6 +27,7 @@ public:
 
 private:
     int _x;
+
     static void print_function(void * self, nix_printer * printer) {}
 
     static void show_type_function(void * self, nix_string_return * res) {}
@@ -68,4 +69,4 @@ TEST_F(nix_api_expr_test, nix_expr_eval_external)
     nix_state_free(stateFn);
 }
 
-}
+} // namespace nixC