Restrict iter.rev() to tests

[herr-seppia]

Summary

The presence of iter().rev() primarily serves debugging purposes. This is because, in difficulty derivation, testing against trailing zeros is more user-friendly, as exemplified by let b: [u8; 2] = [0b11110000, 0] rather than let b: [u8; 2] = [0, 0b11110000].

Possible solution design or implementation

removing iter().rev() within the library and restricting it solely to tests.

Additional context

N/A