Normalize all string inputs according to a particular Unicode rule

[cipriancraciun]

At the moment, any user-provided string inputs (i.e. passwords, additional data, etc.) is used as is, without any form of Unicode normalization.

In order to ensure security and portability, all input strings (that are a superset of ASCII) should be normalized according to a canonical rule.

See the following two issues which stemmed this issue:

• https://github.com/FiloSottile/age/discussions/601
• https://github.com/samuel-lucas6/Kryptor/discussions/78

[cipriancraciun]

• https://datatracker.ietf.org/doc/html/rfc8265 specifies NFC;
• https://pages.nist.gov/800-63-3/sp800-63b.html specifies NFKC or NFKD;
• use https://crates.io/crates/unicode-normalization;