remove InPlaceIterable marker from Peekable due to unsoundness

The unsoundness is not in Peekable per se, it rather is due to the
interaction between Peekable being able to hold an extra item
and vec::IntoIter's clone implementation shortening the allocation.

An alternative solution would be to change IntoIter's clone implementation
to keep enough spare capacity available.

Author: the8472

alloc/benches/vec.rs

@@ -468,7 +468,6 @@ fn bench_in_place_recycle(b: &mut Bencher) {
                 .enumerate()
                 .map(|(idx, e)| idx.wrapping_add(e))
                 .fuse()
-                .peekable()
                 .collect::<Vec<usize>>(),
         );
     });

alloc/tests/vec.rs

@@ -1002,7 +1002,6 @@ fn test_from_iter_specialization_with_iterator_adapters() {
         .zip(std::iter::repeat(1usize))
         .map(|(a, b)| a + b)
         .map_while(Option::Some)
-        .peekable()
         .skip(1)
         .map(|e| if e != usize::MAX { Ok(std::num::NonZeroUsize::new(e)) } else { Err(()) });
     assert_in_place_trait(&iter);

core/src/iter/adapters/peekable.rs

@@ -1,4 +1,4 @@
-use crate::iter::{adapters::SourceIter, FusedIterator, InPlaceIterable, TrustedLen};
+use crate::iter::{adapters::SourceIter, FusedIterator, TrustedLen};
 use crate::ops::{ControlFlow, Try};
 
 /// An iterator with a `peek()` that returns an optional reference to the next
@@ -356,6 +356,3 @@ where
         unsafe { SourceIter::as_inner(&mut self.iter) }
     }
 }
-
-#[unstable(issue = "none", feature = "inplace_iteration")]
-unsafe impl<I: InPlaceIterable> InPlaceIterable for Peekable<I> {}