style(tvix): Replace remaining typedefs with using statements

Change (mostly) created via:

  fd . -e hh -e cc | parallel clang-tidy -p ~/builds/nix --fix "'--checks=-*,modernize-use-using'"

Change-Id: I90ad6e64a5399f2005dfb9956074ec51c2c78cea
Reviewed-on: https://cl.tvl.fyi/c/depot/+/1830
Tested-by: BuildkiteCI
Reviewed-by: glittershark <grfn@gws.fyi>

third_party/nix/src/libutil/pool.hh

@@ -36,7 +36,7 @@ class Pool {
 
   /* A function that checks whether an instance of R is still
      usable. Unusable instances are removed from the pool. */
-  typedef std::function<bool(const ref<R>&)> Validator;
+  using Validator = std::function<bool(const ref<R>&)>;
 
  private:
   Factory factory;