x.crypto.chacha20poly1305: small cleans up and improves safety check

[blackshirt]

After some while for looking it back, i think there are some deficiency that can be improved on. Two's main changes on this pr was

• Changes precomp flag to done flag. precomputation phase for deriving keys was already done and stored on the mac_key, enc_key and poly1305 instance set on psiv creation. By changing to done flag to mark this psiv construct as unusable anymore and effectively preventing to call .encrypt or .decrypt. so we can just returns error if it happens, instead of panic.
• Removes out key field. Its not used on any other places. Its already stored on derived keys, so we can safely remove this out.
• Small cleans up by changing hard coded length on vmemset call
• Add safety check on .encrypt and .decrypt call to aware of done flag

Thanks,
Cheers.

[tankf33der]

All passed.

[spytheman]

Thank you @blackshirt 🙇🏻 .