Add XChaCha20 algorithm, using a 64-bit counter to support large files.

[fzxx]

• The key derivation method is consistent with AES-256.
• The registry records the XChaCha20 option.
• Use a 64-bit counter to support very large files.

# AES-256 encryption + encrypt filenames
7z a -ppassword -mhe=on archive.7z

# XChaCha20 encryption + encrypt filenames
7z a -ppassword -mhe=on -mem=xchacha20 archive.7z

[fzxx]

I noticed that the 7z password derivation does not use a random salt. This is a security issue, and I wonder why. If it's because of speed, I suggest using the standard PBKDF2, which also makes it easy to add a salt. If anyone has a need for MAC, I can also add XChaCha20-Poly1305. @ip7z