Improve Open doc

Part of #29356.

r? @steveklabnik

Dir entry doc

Part of #29356.

r? @steveklabnik

Fix HashMap's values_mut example to use println!

Fix HashMap's values_mut example to use println!

Add DirBuilder doc examples

r? @steveklabnik

Part of #29329 and of #29356.

Update underscore usage (#34903)

Update VecDeque documentation to specify direction of index 0 (#34920)

I mentioned the direction for all the methods that work with an index

Add HashMap Entry enums examples

Part of #29348.

r? @steveklabnik

doc: add missing pause

document DoubleEndedIterator::next_back

document DoubleEndedIterator::next_back

fixes #34726

Add more docs - mostly warnings - to std::mem::transmute

Fix ABI string docs in reference.md

Rollup of 7 pull requests

- Successful merges: #34965, #34972, #34975, #34976, #34977, #34988, #34989
- Failed merges:

Fix incorrect 'memory leak' example for `Vec::set_len`.

Example was written in https://github.com/rust-lang/rust/pull/34911

Issue was brought up in this comment:

https://github.com/rust-lang/rust/commit/a005b2cd2ac679da7393e537aa05e2b7d32d36d5#commitcomment-18346958

Doc example improvements for `slice::windows`.

* Modify existing example to not rely on printing to see results
* Add an example demonstrating when slice is shorter than `size`

Fixed to spelling errors in char.rs

Fixed two small spelling mistakes (interator -> iterator) in the documentation for encode_utf8 and encode_utf16

Add BuildHasher example

r? @steveklabnik

Add Random state doc

Last part of #29348.

r? @steveklabnik

improve const eval error reporting on "" and b"" casts

r? @EddyB

cc @ubsan

Remove unused methods from MultiSpan

Removed a couple of unused methods from MultiSpan.  I thought about batching this with some other changes but wasn't sure when I'd get around to them, so PR for a tiny fix instead.

This can be rollup'd.

Implement rust_eh_personality in Rust, remove rust_eh_personality_catch.

Well, not quite: ARM EHABI platforms still use the old scheme -- for now.

r? @alexcrichton

* Modify existing example to not rely on printing to see results
* Add an example demonstrating when slice is shorter than `size`

Example was written in https://github.com/rust-lang/rust/pull/34911

Issue was brought up in this comment:

https://github.com/rust-lang/rust/commit/a005b2cd2ac679da7393e537aa05e2b7d32d36d5#commitcomment-18346958

Support nested `macro_rules!`

Fixes #6994.
r? @EddyB

Well, not quite: ARM EHABI platforms still use the old scheme -- for now. 

Added empty CloseDelim to tokens for future use.

Description says it all. I added a new DelimToken type, Empty, to indicate a Delimited tokenstream with no actual delimiters (which are variously useful for constructing macro output).

r? @nrc

Fixed two small spelling mistakes (interator -> iterator) in the documentation for encode_utf8 and encode_utf16

Fix wrong condition in base::internalize_symbols().

Fix a typo that snuck into https://github.com/rust-lang/rust/pull/34899 (and completely broke `internalize_symbols()`).

Add method `String::insert_str`

Add a method to the mpsc::Receiver for producing a non-blocking iterator

Currently, the `mpsc::Receiver` offers methods for receiving values in both blocking (`recv`) and non-blocking (`try_recv`) flavours. However only blocking iteration over values is supported. This PR adds a non-blocking iterator to complement the `try_recv` method, just as the blocking iterator complements the `recv` method.

Use-case
-------------

I predominantly use rust in my work on real-time systems and in particular real-time audio generation/processing. I use `mpsc::channel`s to communicate between threads in a purely non-blocking manner. I.e. I might send messages from the GUI thread to the audio thread to update the state of the dsp-graph, or from the audio thread to the GUI thread to display the RMS of each node. These are just a couple examples (I'm probably using 30+ channels across my various projects). I almost exclusively use the `mpsc::Receiver::try_recv` method to avoid blocking any of the real-time threads and causing unwanted glitching/stuttering. Now that I mention it, I can't think of a single time that I personally have used the `recv` method (though I can of course see why it would be useful, and perhaps the common case for many people).

As a result of this experience, I can't help but feel there is a large hole in the `Receiver` API.

| blocking | non-blocking |
|------------|--------------------|
| `recv` | `try_recv` |
| `iter` | 🙀   |

For the most part, I've been working around this using `while let Ok(v) = r.try_recv() { ... }`, however as nice as this is, it is clearly no match for the Iterator API.

As an example, in the majority of my channel use cases I only want to check for *n* number of messages before breaking from the loop so that I don't miss the audio IO callback or hog the GUI thread for too long when an unexpectedly large number of messages are sent. Currently, I have to write something like this:

```rust
let mut take = 100;
while let Ok(msg) = rx.try_recv() {
    // Do stuff with msg
    if take == 0 {
        break;
    }
    take -= 1;
}
```

or wrap the `try_recv` call in a `Range<usize>`/`FilterMap` iterator combo.

On the other hand, this PR would allow for the following:

```rust
for msg in rx.try_iter().take(100) {
    // Do stuff with msg
}
```

I imagine this might also be useful to game devs, embedded or anyone doing message passing across real-time threads.