Rollup merge of rust-lang#84248 - calebsander:refactor/vec-functions, r=Amanieu

Remove duplicated fn(Box<[T]>) -> Vec<T>

`<[T]>::into_vec()` does the same thing as `Vec::from::<Box<[T]>>()`, so they can be implemented in terms of each other. This was the previous implementation of `Vec::from()`, but was changed in rust-lang#78461. I'm not sure what the rationale was for that change, but it seems preferable to maintain a single implementation.

Author: JohnTitor

alloc/src/vec/mod.rs

@@ -2810,8 +2810,7 @@ impl<T, A: Allocator> From<Box<[T], A>> for Vec<T, A> {
     /// assert_eq!(Vec::from(b), vec![1, 2, 3]);
     /// ```
     fn from(s: Box<[T], A>) -> Self {
-        let len = s.len();
-        Self { buf: RawVec::from_box(s), len }
+        s.into_vec()
     }
 }