[#235] Refactor progress bar interface

[aeqz]

Description

Problem: the current progress bar interface is quite low-level and error-prone.

Solution: the proposed solution here modifies both the progress bar internal model and interface by representing progress units by witnesses. It does not require great changes, but the resulting interface is clearer and less error-prone.

Related issue(s)

Relates #235

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Related changes

• Tests

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[YuriRomanowski]

I like how you changed the Progress fields and I like your trick with Set :) I think this is cool, approved.