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/wallet/db.cpp

@@ -102,7 +102,7 @@ bool IsBDBFile(const fs::path& path)
     if (ec) LogWarning("Error reading file_size: %s [%s]", ec.message(), fs::PathToString(path));
     if (size < 4096) return false;
 
-    std::ifstream file{path, std::ios::binary};
+    std::ifstream file{path.std_path(), std::ios::binary};
     if (!file.is_open()) return false;
 
     file.seekg(12, std::ios::beg); // Magic bytes start at offset 12
@@ -126,7 +126,7 @@ bool IsSQLiteFile(const fs::path& path)
     if (ec) LogWarning("Error reading file_size: %s [%s]", ec.message(), fs::PathToString(path));
     if (size < 512) return false;
 
-    std::ifstream file{path, std::ios::binary};
+    std::ifstream file{path.std_path(), std::ios::binary};
     if (!file.is_open()) return false;
 
     // Magic is at beginning and is 16 bytes long

src/wallet/dump.cpp

@@ -134,7 +134,7 @@ bool CreateFromDump(const ArgsManager& args, const std::string& name, const fs::
         error = strprintf(_("Dump file %s does not exist."), fs::PathToString(dump_path));
         return false;
     }
-    std::ifstream dump_file{dump_path};
+    std::ifstream dump_file{dump_path.std_path()};
 
     // Compute the checksum
     HashWriter hasher{};

src/wallet/test/init_test_fixture.cpp

@@ -36,7 +36,7 @@ InitWalletDirTestingSetup::InitWalletDirTestingSetup(const ChainType chainType)
     fs::create_directories(m_walletdir_path_cases["default"]);
     fs::create_directories(m_walletdir_path_cases["custom"]);
     fs::create_directories(m_walletdir_path_cases["relative"]);
-    std::ofstream f{m_walletdir_path_cases["file"]};
+    std::ofstream f{m_walletdir_path_cases["file"].std_path()};
     f.close();
 }