Initial vendor packages

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

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+# proc-macro2
+
+[<img alt="github" src="https://img.shields.io/badge/github-dtolnay/proc--macro2-8da0cb?style=for-the-badge&labelColor=555555&logo=github" height="20">](https://github.com/dtolnay/proc-macro2)
+[<img alt="crates.io" src="https://img.shields.io/crates/v/proc-macro2.svg?style=for-the-badge&color=fc8d62&logo=rust" height="20">](https://crates.io/crates/proc-macro2)
+[<img alt="docs.rs" src="https://img.shields.io/badge/docs.rs-proc--macro2-66c2a5?style=for-the-badge&labelColor=555555&logo=docs.rs" height="20">](https://docs.rs/proc-macro2)
+[<img alt="build status" src="https://img.shields.io/github/actions/workflow/status/dtolnay/proc-macro2/ci.yml?branch=master&style=for-the-badge" height="20">](https://github.com/dtolnay/proc-macro2/actions?query=branch%3Amaster)
+
+A wrapper around the procedural macro API of the compiler's `proc_macro` crate.
+This library serves two purposes:
+
+- **Bring proc-macro-like functionality to other contexts like build.rs and
+  main.rs.** Types from `proc_macro` are entirely specific to procedural macros
+  and cannot ever exist in code outside of a procedural macro. Meanwhile
+  `proc_macro2` types may exist anywhere including non-macro code. By developing
+  foundational libraries like [syn] and [quote] against `proc_macro2` rather
+  than `proc_macro`, the procedural macro ecosystem becomes easily applicable to
+  many other use cases and we avoid reimplementing non-macro equivalents of
+  those libraries.
+
+- **Make procedural macros unit testable.** As a consequence of being specific
+  to procedural macros, nothing that uses `proc_macro` can be executed from a
+  unit test. In order for helper libraries or components of a macro to be
+  testable in isolation, they must be implemented using `proc_macro2`.
+
+[syn]: https://github.com/dtolnay/syn
+[quote]: https://github.com/dtolnay/quote
+
+## Usage
+
+```toml
+[dependencies]
+proc-macro2 = "1.0"
+```
+
+The skeleton of a typical procedural macro typically looks like this:
+
+```rust
+extern crate proc_macro;
+
+#[proc_macro_derive(MyDerive)]
+pub fn my_derive(input: proc_macro::TokenStream) -> proc_macro::TokenStream {
+    let input = proc_macro2::TokenStream::from(input);
+
+    let output: proc_macro2::TokenStream = {
+        /* transform input */
+    };
+
+    proc_macro::TokenStream::from(output)
+}
+```
+
+If parsing with [Syn], you'll use [`parse_macro_input!`] instead to propagate
+parse errors correctly back to the compiler when parsing fails.
+
+[`parse_macro_input!`]: https://docs.rs/syn/2.0/syn/macro.parse_macro_input.html
+
+## Unstable features
+
+The default feature set of proc-macro2 tracks the most recent stable compiler
+API. Functionality in `proc_macro` that is not yet stable is not exposed by
+proc-macro2 by default.
+
+To opt into the additional APIs available in the most recent nightly compiler,
+the `procmacro2_semver_exempt` config flag must be passed to rustc. We will
+polyfill those nightly-only APIs back to Rust 1.56.0. As these are unstable APIs
+that track the nightly compiler, minor versions of proc-macro2 may make breaking
+changes to them at any time.
+
+```
+RUSTFLAGS='--cfg procmacro2_semver_exempt' cargo build
+```
+
+Note that this must not only be done for your crate, but for any crate that
+depends on your crate. This infectious nature is intentional, as it serves as a
+reminder that you are outside of the normal semver guarantees.
+
+Semver exempt methods are marked as such in the proc-macro2 documentation.
+
+<br>
+
+#### License
+
+<sup>
+Licensed under either of <a href="LICENSE-APACHE">Apache License, Version
+2.0</a> or <a href="LICENSE-MIT">MIT license</a> at your option.
+</sup>
+
+<br>
+
+<sub>
+Unless you explicitly state otherwise, any contribution intentionally submitted
+for inclusion in this crate by you, as defined in the Apache-2.0 license, shall
+be dual licensed as above, without any additional terms or conditions.
+</sub>