Refactor __str__ methods for readability and consistency

[LexxXell]

Refactor: modernize and unify __str__ methods across transaction classes

This PR updates the __str__ implementations for several transaction-related classes to improve readability, consistency, and maintainability while preserving exact output semantics.

Summary of changes

• Replaced old % formatting with f-strings for clarity and modern Python style.
• Preserved all original field order, line breaks, and output format to avoid breaking GUI or existing log parsers.
• Kept binary-to-hex conversions (bh2u) exactly as before where originally present.
• Removed outdated formatting artifacts (typos such as sequeence).
• Ensured that no functional logic or serialization behavior is modified — only string representation.

Affected components

• CTxIn
• CTxOut
• ProRegTx
• ProUpRegTx
• ProUpRevTx
• CbTx
• AssetLockTx
• AssetUnlockTx

Why this change

The original __str__ implementations were inconsistent in style and used legacy formatting.
This cleanup makes the codebase more uniform and maintainable while ensuring output remains fully backward-compatible.

Compatibility

✔ No behavior changes
✔ No serialization logic changes
✔ Output format preserved
✔ Safe for all downstream consumers (GUI/logs/tests)