Performance enhancement for directory iteration on POSIX

[gulrak]

This ticket collects some aspects that lead to lower performance on directory iteration on POSIX backend (e.g. macOS, Linux and FreeBSD).

• fs::status doesn't always fill symlink information when possible, leading to subsequent redundant ::lstat calls.
• fs::directory_iterator always recreates fs::directory_entry even when there is fs::directory_entry::assign
• fs::recursive_directory_iterator queries full status and symlink status even when the only question is it the type is directory or symlink.

[gulrak]

This is now part of release v1.5.2