Auto merge of rust-lang#61953 - Centril:shared-from-iter, r=RalfJung

Add `impl<T> FromIterator<T> for Arc/Rc<[T]>`

Add implementations of `FromIterator<T> for Arc/Rc<[T]>` with symmetrical logic.

This also takes advantage of specialization in the case of iterators with known length (`TrustedLen`) to elide the final allocation/copying from a `Vec<T>` into `Rc<[T]>` because we can allocate the space for the `Rc<[T]>` directly when the size is known. This is the primary motivation and why this is to be preferred over `iter.collect::<Vec<_>>().into(): Rc<[T]>`.

Moreover, this PR does some refactoring in some places.

r? @RalfJung for the code
cc @alexcrichton from T-libs

Author: bors

src/liballoc/lib.rs

@@ -93,6 +93,7 @@
 #![feature(ptr_offset_from)]
 #![feature(rustc_attrs)]
 #![feature(receiver_trait)]
+#![feature(slice_from_raw_parts)]
 #![feature(specialization)]
 #![feature(staged_api)]
 #![feature(std_internals)]