Move to stable

[skade]

Hi,

this patch fixes coreutils to compile on stable Rust by mostly following the recommendations given when removing the feature flags. In detail, that is:

• remove fmts dependency on rustc_unicode. It wasn't using it anyways.
• Remove CharRange usage from fmt and use the recommended handling methods (char_indices and chars). These methods recently got deprecated, see Tracking issue for str_char stabilization rust-lang/rust#27754, the struct is not committed to.
• Remove usage of io::Read::chars() in tr. The method is still under debate (Tracking issue for Read::chars rust-lang/rust#27802). As a buffered reader is used anyways, I concluded that we could just read into a buffer and work on that. I'm rather unsure if this patch is super-clean, it's a bit late. All tests pass and I think logically, it's alright, I'm just pretty sure it could be expressed neater.

Best,
Florian, rust-community team

[jbcrail]

This looks great, and the code is clearer. 👍 Thanks!