crypto::chacha provides an implementation of the Chacha20 and XChacha20 stream
ciphers.

Use [[chacha20]] to create a [[crypto::cipher::xorstream]] and either
[[chacha20_init]] or [[xchacha20_init]] to set the handle, key and nonce of the
appropriate size, [[NONCESIZE]] for chacha20 or [[XNONCESIZE]] for XChacha20.
After calling the appropriate init function, [[io::write]] may be used to
encrypt blocks to the handle or [[io::read]] to decrypt blocks from the handle.
The stream must be closed with [[io::close]] to wipe sensitive data from
memory.

Writing blocks of length [[BLOCKSIZE]] is not required. However, seeking the
key stream with [[setctr]] only operates in units of [[BLOCKSIZE]].

This is a low-level module which implements cryptographic primitives. Direct
use of cryptographic primitives is not recommended for non-experts, as
incorrect use of these primitives can easily lead to the introduction of
security vulnerabilities. Non-experts are advised to use the high-level
operations available in the top-level [[crypto]] module.

Be advised that Hare's cryptography implementations have not been audited.
