xchacha: refactor to remove dead code (#2)

* chacha: drop internal HChaCha20.

* chacha: remove more dead code.

* xchacha: rename.

* chacha: merge packages.

* chacha: more dead code.

* chacha: merge into xchacha.

* xchacha: rename.

* xchacha: fix panics.

Merge pull request #1 from ncruces/dev

Improve throughput on arm64