Vendor dependencies for 0.3.0 release

vendor/memmap2/README.md

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+# memmap2
+![Build Status](https://github.com/RazrFalcon/memmap2-rs/workflows/Rust/badge.svg)
+[![Crates.io](https://img.shields.io/crates/v/memmap2.svg)](https://crates.io/crates/memmap2)
+[![Documentation](https://docs.rs/memmap2/badge.svg)](https://docs.rs/memmap2)
+[![MSRV 1.63.0](https://img.shields.io/badge/msrv-1.63.0-dea584.svg?logo=rust)](https://github.com/rust-lang/rust/releases/tag/1.63.0)
+
+A Rust library for cross-platform memory mapped IO.
+
+This is a **fork** of the [memmap-rs](https://github.com/danburkert/memmap-rs) crate.
+
+## Features
+
+- [x] file-backed memory maps
+- [x] anonymous memory maps
+- [x] synchronous and asynchronous flushing
+- [x] copy-on-write memory maps
+- [x] read-only memory maps
+- [x] stack support (`MAP_STACK` on unix)
+- [x] executable memory maps
+- [x] huge page support (linux only)
+
+A list of supported/tested targets can be found in [Actions](https://github.com/RazrFalcon/memmap2-rs/actions).
+
+## License
+
+`memmap2` is primarily distributed under the terms of both the MIT license and the
+Apache License (Version 2.0).
+
+See [LICENSE-APACHE](LICENSE-APACHE), [LICENSE-MIT](LICENSE-MIT) for details.
+
+Copyright (c) 2020 Yevhenii Reizner
+
+Copyright (c) 2015 Dan Burkert