Remove the Keccak C library and use the pure Rust impl (#8657)

* Add license and readme

* Use pure rust implementation

* Bump version to 0.1.1

* Don't use C, prefer the pure Rust implementation

* Add test for `write_keccak`

* Bump version

* Add benchmarks

* Add benchmarks

* Add keccak_256, keccak_512, keccak_256_unchecked and keccak_512_unchecked – mostly for compatibility with ethash

* Remove failed git merge attempt from external git repo
Cargo.lock updates

* whitespace

* Mark unsafe function unsafe

* Unsafe calls in unsafe block

* Document unsafety invariants

* Revert unintended changes to Cargo.lock