Add setting to warn about copying/hashing large paths

This is useful for diagnosing whether an evaluation is copying large
paths to the store. Example:

   $ nix build .#packages.x86_64-linux.default --large-path-warning-threshold 1000000
   warning: copied large path '/home/eelco/Dev/nix-master/' to the store (6271792 bytes)
   warning: copied large path '«github:NixOS/nixpkgs/b550fe4b4776908ac2a861124307045f8e717c8e?narHash=sha256-7kkJQd4rZ%2BvFrzWu8sTRtta5D1kBG0LSRYAfhtmMlSo%3D»/' to the store (155263768 bytes)
   warning: copied large path '«github:libgit2/libgit2/45fd9ed7ae1a9b74b957ef4f337bc3c8b3df01b5?narHash=sha256-oX4Z3S9WtJlwvj0uH9HlYcWv%2Bx1hqp8mhXl7HsLu2f0%3D»/' to the store (22175416 bytes)
   warning: copied large path '/nix/store/z985088mcd6w23qwdlirsinnyzayagki-source' to the store (5885872 bytes)

src/libstore/globals.hh

@@ -1262,6 +1262,16 @@ public:
           store paths of the latest Nix release.
         )"
     };
+
+    Setting<uint64_t> largePathWarningThreshold{
+        this,
+        std::numeric_limits<uint64_t>::max(),
+        "large-path-warning-threshold",
+        R"(
+          Warn when copying a path larger than this number of bytes to the Nix store
+          (as determined by its NAR serialisation).
+        )"
+    };
 };