Formatted with cargo clippy and removed redundant code

[TadaHrd]

• #[doc(hidden)] is redundant on private items unless you compile docs with --document-private-items
• redundant lifetimes in Hotp and Totp methods.
• todo!() in generate_otpauth_url
• number operations like x / 1 or x & 0xFF removed in favor of *x.
• assert_eq!(x, true) replaced with assert!(x), same with false.

There are still things that I haven't done that could be done:

• set the edition to 2021, it should be compatible
• use debug_assert to optimize for release builds
• add explanations into assertions (e.g. assert!(cond, "cond was false, that shouldn't happen"))
• add support for otpauth URL generation (I could make a PR if you wanted)
• put generate_otpauth_url behind a feature in the Cargo.toml file

Note:

• the secret should be in base32, at least for otpauth URLs
• I did not bump the version in Cargo.toml