add simplified memo utilities for Hash and Return memo types

[DokaIzk]

• Add buildHashMemo() and buildReturnMemo() utilities for simplified
  memo construction from Buffer, Uint8Array, or hex strings
• Add comprehensive validation with MemoValidationError custom class
• Add memoToHex() for conversion to hex representation
• Add hexToMemoBuffer() for safe hex to buffer conversion
• Add compareMemos() for equality comparison
• Include full TypeScript and JavaScript implementations
• Add 50+ test cases covering edge cases and integration paths

Closes #159

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[DokaIzk]

@Solomonsolomonsolomon kindly awaiting your review