Opinionated refactoring of #16415

[mrmr1993]

This PR builds upon #16415. The commits in this PR apply a series of transformations to make the code more readable and maintainable. Most notably:

• the pervasive use of the overloaded word 'spec' is removed
• Account_spec is renamed to Simple_account
• the Account_spec.t array pattern is encapsulated as a new Simple_ledger module
• Command_spec is renamed to Simple_command
• single-letter and non-descriptive variables names have been updated
• functions and generators that cause side-effects on ledgers are named to record that
• the edge-case generators are split into their own helper functions
• transactions now carry a sender_info struct instead of a full account
• yojson is used in place of s-expression strings masquerading as JSON
• comment formatting is fixed
• some inline functions are lifted outside as appropriate.

The goal of this PR is to modify the original so that abstractions and naming guide the reader towards an understanding of mutability, the differences between the various arrays and 'spec' concepts, and more generally the mapping between the test code and the concepts in the rest of the code. The diff with compatible can be found here for reference.

[mrmr1993]

!ci-build-me

[dkijania]

changes in 2372a97 and caa9b60 also look ok