Prior to this commit, `str` documented that `get_unchecked` had
the precondition that "`begin` must come before `end`". This would appear
to prohibit empty slices (i.e. begin == end).

In practice, get_unchecked is called often with empty slices. Let's relax
the precondition so as to allow them.

Suggested by @ametisf in rust-lang#65948 (comment)

Co-Authored-By: Frantisek Fladung <fladufra@fel.cvut.cz>

… r=nagisa

Relax str::get_unchecked precondition to permit empty slicing

Prior to this commit, `str` documented that `get_unchecked` had
the precondition that "`begin` must come before `end`". This would appear
to prohibit empty slices (i.e. begin == end).

In practice, get_unchecked is called often with empty slices. Let's relax
the precondition so as to allow them.

…cs_replace_chunk_with_windows, r=Dylan-DPC

Fix minor error in `MaybeUninit::get_mut()` doc example

In the `MaybeUninit::get_mut()` example I wanted to assert that the slice was sorted and mistakenly used `.chunks(2)` rather than `.windows(2)` to assert it, as @ametisf pointed out in rust-lang#65948 (comment) .

This fixes it.

…crum

Add rustdoc aliases to `ptr::copy` and `ptr::copy_nonoverlapping`

This PR adds a [rustdoc alias](https://doc.rust-lang.org/nightly/rustdoc/unstable-features.html#add-aliases-for-an-item-in-documentation-search) to `ptr::copy` and `ptr::copy_nonoverlapping` using the commonly used terms `memcpy` and `memmove`. The motivation for this PR is to improve discoverability for these functions, since I've noticed users overlook these functions on multiple occasions (and they have thus reached for [libc](https://crates.io/crates/libc) without need). Currently std docs state:

https://doc.rust-lang.org/nightly/std/ptr/fn.copy_nonoverlapping.html
> `copy_nonoverlapping` is semantically equivalent to C's `memcpy`, but with the argument order swapped.

https://doc.rust-lang.org/nightly/std/ptr/fn.copy.html
> `copy` is semantically equivalent to C's `memmove`, but with the argument order swapped.

#### search results before adding a rustdoc alias:
![screenshot 6517](https://user-images.githubusercontent.com/6868531/75102985-78fbb680-55c2-11ea-8e41-04979e6fa6f6.png)
![screenshot 6518](https://user-images.githubusercontent.com/6868531/75102984-78632000-55c2-11ea-9673-8822aae636d1.png)

#### after adding `#[doc(alias = "memcpy")]` and `#[doc(alias = "memmove")]`:
![screenshot 6516](https://user-images.githubusercontent.com/6868531/75102986-78fbb680-55c2-11ea-93b9-1929be940043.png)
![screenshot 6515](https://user-images.githubusercontent.com/6868531/75102987-78fbb680-55c2-11ea-9861-ce8a77a0c3b9.png)

…an-DPC

Clean up E0367 explanation

r? @Dylan-DPC