Feat: Add clippy stackslib, pass indexing_slicing

[kantai]

This gets stackslib to pass the first lint listed in #6156

This does a couple of things in the refactor beyond just replacing foo[..] with foo.get(..)? in order to make the usage a little more idiomatic in rust:

• More usage of TryInto<[u8; T]> -- we previously did a lot of buffering in our codebase to get from &[u8] into a [u8; T]. This not only involves indexing or invocations of copy_from_slice(), but it also is less performant.
• More reliance on iterator operations like .take(), .chunks(), .zip(), and .skip(), etc.

Some of the previous avoidance of iterators (in MARF in particular) had comments indicating that iterators were slower than the alternatives (for loops, and even manual for loops). I'm not sure if that used to be the case in older versions of rustc, but as of this PR, there was no measurable difference when measured using the MARF benchmarks (with optimizations turned on: I think in unoptimized builds, there is a performance difference).