Refactor - Script stores bytes, not strings

[DaMatrix]

Description

This PR depends on #42 (placeholders).

Our scripts can contain "byte array" constants, however they are actually represented as UTF-8 strings which in practice always contain hexadecimal data. This is not only inefficient, but also means that many of the stack operations behave in unexpected ways (e.g. the indices passed to OP_SUBSTR are effectively in units of half-bytes rather than bytes). This PR changes them into actual byte vectors, although for now they are still temporarily converted to hex strings in order to emulate the original behavior when performing cryptographic operations on the stack. In the future (preferably before all of these refactor PRs get merged), it would be ideal if this hex string translation could be eliminated, as it will likely be a major source of bugs for other developers attempting to work with our system.

Changelog

• Changed StackEntry::Bytes to contain a Vec<u8> instead of a String
• Updated all code which uses StackEntry::Bytes from hex strings to byte arrays, as necessary

Type of Change

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• Bug fix
• New feature
• Enhancement or optimization
• Documentation update
• Other (please specify)

Checklist

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• I have tested the changes locally and they work as expected.
• I have added necessary documentation or updated existing documentation.
• My code follows the project's coding standards and style guidelines.
• I have added/updated relevant tests to ensure the changes are properly covered.
• I have checked for and resolved any merge conflicts.
• My commits have clear and descriptive messages.

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[BHouwens]

This one will require testing against existing chain data to ensure you've got all the edge cases in handling legacy stacks. Otherwise, looks all good