Fix windows libc++ fs::path fstream compile errors

As reported by hebasto in https://github.com/bitcoin/bitcoin/issues/33545,
newer libc++ versions implementing https://wg21.link/lwg3430 will no longer
implicitly convert `fs::path` objects to `std::filesystem::path` objects when
constructing `std::ifstream` and `std::ofstream` types.

This is not a problem in Unix systems since `fs::path` objects use
`std::string` as their native string type, but it causes compile errors on
Windows which use `std::wstring` as their string type, since `fstream`s can't
be constructed from `wstring`s.

Fix the windows libc++ compile errors by adding a new `fs::path::std_path()`
method and using it construct `fstream`s more portably.

Additionally, delete `fs::path`'s implicit `native_string` conversion so these
errors will not go undetected in the future, even though there is not currently
a CI job testing Windows libc++ builds.

src/test/script_assets_tests.cpp

@@ -159,7 +159,7 @@ BOOST_AUTO_TEST_CASE(script_assets_test)
     bool exists = fs::exists(path);
     BOOST_WARN_MESSAGE(exists, "File $DIR_UNIT_TEST_DATA/script_assets_test.json not found, skipping script_assets_test");
     if (!exists) return;
-    std::ifstream file{path};
+    std::ifstream file{path.std_path()};
     BOOST_CHECK(file.is_open());
     file.seekg(0, std::ios::end);
     size_t length = file.tellg();