This commit applies all stabilizations, renamings, and deprecations that the
library team has decided on for the upcoming 1.9 release. All tracking issues
have gone through a cycle-long "final comment period" and the specific APIs
stabilized/deprecated are:
* `std::raw::Slice` - use raw parts of `slice` module instead
* `std::raw::Repr` - use raw parts of `slice` module instead
* `str::char_range_at` - use slicing plus `chars()` plus `len_utf8`
* `str::char_range_at_reverse` - use slicing plus `chars().rev()` plus `len_utf8`
* `str::char_at` - use slicing plus `chars()`
* `str::char_at_reverse` - use slicing plus `chars().rev()`
* `str::slice_shift_char` - use `chars()` plus `Chars::as_str`
* `CommandExt::session_leader` - use `before_exec` instead.
Closes #27719
cc #27751 (deprecating the `Slice` bits)
Closes #27754
Closes #27780
Closes #27809
Closes #27811
Closes #27830
Closes #28050
Closes #29453
Closes #29791
Closes #29935
Closes #30014
Closes #30752
Closes #31262
cc #31398 (still need to deal with `before_exec`)
Closes #31405
Closes #31572
Closes #31755
Closes #31756