Address more clippy lints

[rillian]

Fix style issues with the Rust code to reduce the lint noise and make more serious issues easier to notice

• Remove redundant casts and use integer literals to specify types from initializers
• Remove unnecessary borrows when the type was already a reference or didn't need to be mutable.
• Mark two functions triggering the "too many arguments" lint as expected.
• Refactor GCM::hex2bytes to use range patterns

Follow up to #70. This still isn't everything, but I felt it was useful progress.