Seal DirBuilderExt, DirEntryExt, and OpenOptionsExt #10
Description
unix::DirBuilderExt, unix::DirEntryExt, unix::OpenOptionsExt, and windows::OpenOptionsExt are not intended to be implemented by users, and they should be sealed.
Note that this is a breaking change. (However, given the standard library added a new method in the minor version to os-specific extension traits (not sealed) (1, 2), we may not consider this is a breaking change that requires a major version bump.)
Related: smol-rs/async-process#6, f4cc6e5#commitcomment-42948266