Replace Split*::as_str with remainder

This commit
- Renames `Split*::{as_str -> remainder}` as it seems less confusing
- Makes `remainder` return Option<&str> to distinguish between
  "iterator is exhausted" and "the tail is empty"

library/core/src/lib.rs

@@ -158,6 +158,7 @@
 #![feature(const_unsafecell_get_mut)]
 #![feature(const_waker)]
 #![feature(core_panic)]
+#![feature(char_indices_offset)]
 #![feature(duration_consts_float)]
 #![feature(maybe_uninit_uninit_array)]
 #![feature(ptr_alignment_type)]
@@ -166,6 +167,8 @@
 #![feature(slice_ptr_get)]
 #![feature(slice_split_at_unchecked)]
 #![feature(str_internals)]
+#![feature(str_split_remainder)]
+#![feature(str_split_inclusive_remainder)]
 #![feature(strict_provenance)]
 #![feature(utf16_extra)]
 #![feature(utf16_extra_const)]