Implement Word struct wrapper for [u8; WORD_LENGTH] with base64 Display

[Copilot]

This PR implements a Word<const WORD_LENGTH: usize> wrapper struct around [u8; WORD_LENGTH] that provides base64 display functionality, as suggested in the original discussion.

Changes

New Word Struct

• Added Word<const WORD_LENGTH: usize>([u8; WORD_LENGTH]) wrapper struct
• Implements Display trait that shows base64 encoding of the wrapped byte array
• Provides clean, readable output for debugging and logging

Conversion Traits

To ensure seamless integration with existing code, the following traits are implemented:

• From<[u8; WORD_LENGTH]> and From<Word<WORD_LENGTH>> for easy conversion
• AsRef<[u8; WORD_LENGTH]> and AsRef<[u8]> for reference access
• Deref to [u8; WORD_LENGTH] for transparent usage

Usage Example

use cosmian_findex_structs::Word;

let word_bytes = [72, 101, 108, 108, 111, 32, 87, 111, 114, 108, 100, 33]; // "Hello World!"
let word: Word<12> = Word::new(word_bytes);

println!("{}", word); // Outputs: SGVsbG8gV29ybGQh (base64)

// Seamless conversion back to byte array
let bytes: [u8; 12] = word.into();

Compatibility

The implementation maintains full backward compatibility:

• Existing [u8; WORD_LENGTH] usage continues to work unchanged
• Zero-cost abstractions through the Deref trait
• Type safety preserved with compile-time size guarantees

Testing

Added comprehensive tests covering:

• Base64 display functionality with known test vectors
• All conversion trait implementations
• Round-trip conversions
• Integration with existing CUSTOM_WORD_LENGTH constant

Fixes #94.

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