Consolidate panicking functions in slice/index.rs

Consolidate all the panicking functions in `slice/index.rs` to use a single
`slice_index_fail` function, similar to how it is done in `str/traits.rs`.

Author: Kmeakin

tests/panic/oob_subslice.stderr

@@ -1,5 +1,5 @@
 
 thread 'main' ($TID) panicked at tests/panic/oob_subslice.rs:LL:CC:
-range end index 5 out of range for slice of length 4
+range end index 4 out of range for slice of length 4
 note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
 note: in Miri, you may have to set `MIRIFLAGS=-Zmiri-env-forward=RUST_BACKTRACE` for the environment variable to have an effect