`Path`s and filenames are not necessarily UTF-8

[huonw]

$ mkdir empty
$ cd empty
$ ls
$ touch $(echo -e "\xff") # `ÿ` in latin1, invalid UTF8, but valid filename
$ ls
?
$ rusti
WARNING: The Rust REPL is experimental and may be
unstable. If you encounter problems, please use the
compiler instead. Type :help for help.
rusti> std::os::list_dir(&Path("."))
rust: task failed at 'assertion failed: is_utf8(v)', /home/huon/rust/src/libstd/str.rs:758
terminate called after throwing an instance of 'rust_task*'
Aborted

Path and all interfaces directly with the file-system should probably use [u8] rather than trying to coerce str to handle these cases. (Similar issue to #7188.)

[bblum]

nominating feature-complete

[catamorphism]

Accepted for backwards-compatible

[lilyball]

My last referenced commit is a month old, but I'm still working on this issue (currently finishing up the support for Windows paths).

[pnkfelix]

The issues here of dealing with filesystems that are not utf8 seem related to #2253, at least tangentially.