Implement async Path & PathBuf

[Wassasin]

Partially closes #183. Tracking PR as quite a lot of methods need to be implemented.

PathBuf

• path::PathBuf::as_path
• path::PathBuf::into_boxed_path
• path::PathBuf::into_os_string
• path::PathBuf::new
• path::PathBuf::pop
• path::PathBuf::push
• path::PathBuf::set_extension
• path::PathBuf::set_file_name

Path

• path::Path::ancestors
• path::Path::as_os_str
• path::Path::canonicalize
• path::Path::components
• path::Path::display
• path::Path::ends_with
• path::Path::exists
• path::Path::extension
• path::Path::file_name
• path::Path::file_stem
• path::Path::has_root
• path::Path::into_path_buf
• path::Path::is_absolute
• path::Path::is_dir
• path::Path::is_file
• path::Path::is_relative
• path::Path::iter
• path::Path::join
• path::Path::metadata
• path::Path::new
• path::Path::parent
• path::Path::read_dir
• path::Path::read_link
• path::Path::starts_with
• path::Path::strip_prefix
• path::Path::symlink_metadata
• path::Path::to_path_buf
• path::Path::to_str
• path::Path::to_string_lossy
• path::Path::with_extension
• path::Path::with_file_name

[Wassasin]

@yoshuawuyts I'm unsure as how to feature gate the unstable path_buf_capacity feature within async-std. The remaining methods depend on this feature.

Also the ancestors method yields and iterator over std::path::Path, instead of our own Path. It may be worth considering mapping this iterator and yielding impl iterator<Item=Path>.

[ghost]

I'm unsure as how to feature gate the unstable path_buf_capacity feature within async-std. The remaining methods depend on this feature.

Let's just omit unstable functions from the standard library.

Also the ancestors method yields and iterator over std::path::Path, instead of our own Path. It may be worth considering mapping this iterator and yielding impl iterator<Item=Path>.

We should define our own Ancestors iterator instead of re-exporting the one from std.

[yoshuawuyts]

Looks great; let's merge this!

[yoshuawuyts]

@Wassasin thanks so much for all your work on this! This is fantastic!