Refactor underlying data structure of assertions AST from string to something more flexible

[farbodahm]

What's the problem this feature will solve?

The current data structure for assertion AST, which contains assertion explanations, is a large chunk of string generated in rewrite.py. This string is then passed to format_explanation for further formatting.

Describe the solution you'd like

Implement a more flexible data structure (such as a tree) to construct the assertion explanations. This structure can then provide the string representation when needed, allowing for greater flexibility and potential improvements in how explanations are handled and formatted.

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