Make `char::is_private_use` and `char::is_assigned` unstably public See also https://github.com/rust-lang/rust/pull/157944, https://github.com/rust-lang/rust/pull/154849. These functions are used in the implementation of `char::escape_debug()`, and there's no reason to force crates to duplicate data tables that std has to ship anyway. `is_assigned` is especially useful because crates relying on the stability guarantees of the other Unicode methods in `char` may wish to reject unassigned characters where those guarantees do not hold. I'll add a tracking issue if this is approved. The `is_assigned` vs `is_unassigned` bikeshed probably needs libs-API review before merging? @rustbot label A-Unicode T-libs-api r? @Mark-Simulacrum