add Vec::extend_from_within method

Implement <https://github.com/rust-lang/rfcs/pull/2714>, changes from the RFC:
- Rename the method `append_from_within` => `extend_from_within`
- Loose :Copy bound => :Clone
- Specialize in case of :Copy

This commit also adds `Vec::split_at_spare` private method and use it to implement
`Vec::spare_capacity_mut` and `Vec::extend_from_within`. This method returns 2
slices - initialized elements (same as `&mut vec[..]`) and uninitialized but
allocated space (same as `vec.spare_capacity_mut()`).

library/alloc/src/lib.rs

@@ -76,6 +76,7 @@
 #![cfg_attr(test, feature(test))]
 #![cfg_attr(test, feature(new_uninit))]
 #![feature(allocator_api)]
+#![feature(vec_extend_from_within)]
 #![feature(array_chunks)]
 #![feature(array_methods)]
 #![feature(array_value_iter)]