align included license files with current Rust project best practices

[decathorpe]

Hi! I'm currently working on packaging this crate for Fedora Linux as a dependency of hashbrown v0.14.0 and bumpalo, and noticed that the setup for license files in this repository is currently quite unusual for a Rust project (and also non-compliant with the Apache-2.0 license, since it requires the full license text to be included, not only the header).

I've made some changes to align this project with other Rust projects and best practices, i.e. renamed license files to LICENSE-APACHE, LICENSE-MIT, and removed the COPYING file which is redundant with the ## License paragraph from the README (and which also included the wrong paths for the Apache-2.0 / MIT license texts).

The end result is equivalent to the setup in the repo for the proc-macro2 crate (which I took as a reference because both proc-macro2 and allocator-api2 are reimplementations of Rust features), and the rust-lang/rust project itself - notable changes include:

• include full Apache-2.0 license text in LICENSE-APACHE (without appendix; see ¹ and ²)
• remove copyright from MIT license (this is an anachronism and a noop; see ³ and ⁴)

---

¹ rust-lang/rust@2ccf65c
² dtolnay/proc-macro2@37e89e1
³ rust-lang/rust@2a8807e
⁴ dtolnay/proc-macro2@f633e31

[zakarumych]

Thank you for the contribution

[decathorpe]

Thanks for merging! 💯