Initial vendor packages

Signed-off-by: Valentin Popov <valentin@popov.link>

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+<p align="center"><img src="design/logo.png" alt="once_cell"></p>
+
+
+[![Build Status](https://github.com/matklad/once_cell/actions/workflows/ci.yaml/badge.svg)](https://github.com/matklad/once_cell/actions)
+[![Crates.io](https://img.shields.io/crates/v/once_cell.svg)](https://crates.io/crates/once_cell)
+[![API reference](https://docs.rs/once_cell/badge.svg)](https://docs.rs/once_cell/)
+
+# Overview
+
+`once_cell` provides two new cell-like types, `unsync::OnceCell` and `sync::OnceCell`. `OnceCell`
+might store arbitrary non-`Copy` types, can be assigned to at most once and provide direct access
+to the stored contents. In a nutshell, API looks *roughly* like this:
+
+```rust
+impl OnceCell<T> {
+    fn new() -> OnceCell<T> { ... }
+    fn set(&self, value: T) -> Result<(), T> { ... }
+    fn get(&self) -> Option<&T> { ... }
+}
+```
+
+Note that, like with `RefCell` and `Mutex`, the `set` method requires only a shared reference.
+Because of the single assignment restriction `get` can return an `&T` instead of `Ref<T>`
+or `MutexGuard<T>`.
+
+`once_cell` also has a `Lazy<T>` type, build on top of `OnceCell` which provides the same API as
+the `lazy_static!` macro, but without using any macros:
+
+```rust
+use std::{sync::Mutex, collections::HashMap};
+use once_cell::sync::Lazy;
+
+static GLOBAL_DATA: Lazy<Mutex<HashMap<i32, String>>> = Lazy::new(|| {
+    let mut m = HashMap::new();
+    m.insert(13, "Spica".to_string());
+    m.insert(74, "Hoyten".to_string());
+    Mutex::new(m)
+});
+
+fn main() {
+    println!("{:?}", GLOBAL_DATA.lock().unwrap());
+}
+```
+
+More patterns and use-cases are in the [docs](https://docs.rs/once_cell/)!
+
+# Related crates
+
+* [double-checked-cell](https://github.com/niklasf/double-checked-cell)
+* [lazy-init](https://crates.io/crates/lazy-init)
+* [lazycell](https://crates.io/crates/lazycell)
+* [mitochondria](https://crates.io/crates/mitochondria)
+* [lazy_static](https://crates.io/crates/lazy_static)
+* [async_once_cell](https://crates.io/crates/async_once_cell)
+* [generic_once_cell](https://crates.io/crates/generic_once_cell) (bring your own mutex)
+
+Parts of `once_cell` API are included into `std` [as of Rust 1.70.0](https://github.com/rust-lang/rust/pull/105587).