Rollup merge of #65479 - SimonSapin:matches, r=alexcrichton

Add the `matches!( $expr, $pat ) -> bool` macro

# Motivation

This macro is:

* General-purpose (not domain-specific)
* Simple (the implementation is short)
* Very popular [on crates.io](https://crates.io/crates/matches) (currently 37th in all-time downloads)
* The two previous points combined make it number one in [left-pad index](https://twitter.com/bascule/status/1184523027888988160) score

As such, I feel it is a good candidate for inclusion in the standard library.

In fact I already felt that way five years ago: https://github.com/rust-lang/rust/pull/14685 (Although the proof of popularity was not as strong at the time.)

# API

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Back then, the main concern was that this macro may not be quite universally-enough useful to belong in the prelude.

Therefore, this PR adds the macro such that using it requires one of:

```rust
use core::macros::matches;
use std::macros::matches;
```

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Like arms of a `match` expression, the macro supports multiple patterns separated by `|` and optionally followed by `if` and a guard expression:

```rust
let foo = 'f';
assert!(matches!(foo, 'A'..='Z' | 'a'..='z'));

let bar = Some(4);
assert!(matches!(bar, Some(x) if x > 2));
```

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# Implementation constraints

A combination of reasons make it tricky for a standard library macro not to be in the prelude.

Currently, all public `macro_rules` macros in the standard library macros end up “in the prelude” of every crate not through `use std::prelude::v1::*;` like for other kinds of items, but through `#[macro_use]` on `extern crate std;`. (Both are injected by `src/libsyntax_ext/standard_library_imports.rs`.)

`#[macro_use]` seems to import every macro that is available at the top-level of a crate, even if through a `pub use` re-export.

Therefore, for `matches!` not to be in the prelude, we need it to be inside of a module rather than at the root of `core` or `std`.

However, the only way to make a `macro_rules` macro public outside of the crate where it is defined appears to be `#[macro_export]`. This exports the macro at the root of the crate regardless of which module defines it. See [macro scoping](https://doc.rust-lang.org/reference/macros-by-example.html#scoping-exporting-and-importing) in the reference.

Therefore, the macro needs to be defined in a crate that is not `core` or `std`.

# Implementation

This PR adds a new `matches_macro` crate as a private implementation detail of the standard library. This crate is `#![no_core]` so that libcore can depend on it. It contains a `macro_rules` definition with `#[macro_export]`.

libcore and libstd each have a new public `macros` module that contains a `pub use` re-export of the macro. Both the module and the macro are unstable, for now.

The existing private `macros` modules are renamed `prelude_macros`, though their respective source remains in `macros.rs` files.

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