Refactor - Placeholders

[DaMatrix]

Description

This adds a pair of traits intended for generating placeholder values, intended for use in tests.

A dedicated trait for placeholder values is preferable to using the standard Default trait for two reasons:

• It allows us to generate multiple unique placeholder values, which are nonetheless consistent across test runs (this is a problem for much of the existing test code, which e.g. extensively uses crypto::gen_keypair() to generate a totally random keypair every time)
• It enables obtaining placeholder values for types for which it would not make sense to allow a default value: e.g. SecretKey clearly should not implement Default, as that would assume the existence of some kind of "default" secret key (which doesn't make sense)

These traits will be extensively used in subsequent PRs.

Changelog

• Added two test-only traits for generating placeholder values:
  • One for generating completely arbitrary placeholder values, with no guarantees made
  • One for generating placeholder values based on a given seed, which guarantees that it will return consistent values across runs

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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