refactor!: Make local_variables a dictionary instead of a list of strings

[dexter2206]

Description

Currently, we store local variables as a list of strings, each of them of the form "symbol = expression", e.g.:

local_variables = ["a = x + y", "b = 42"]

However, each time such variables are processed, this lists of strings gets converted to a dictionary. Therefore, it is only natural to make local_variables a dictionary in the first place, so that the example above looks like:

local_variables = {"a": "x+y", "b": "42"}

which is what this PR does.

While refactoring this, some functions became obsolete and were thus removed from the codebase entirely as a part of this PR.

Please verify that you have completed the following steps

• I have self-reviewed my code.
• I have included test cases validating introduced feature/fix.
• I have updated documentation.

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[cla-bot]

Thank you for your pull request and welcome to our community. We could not parse the GitHub identity of the following contributors: Konrad Jałowiecki.
This is most likely caused by a git client misconfiguration; please make sure to:

1. check if your git client is configured with an email to sign commits git config --list | grep email
2. If not, set it up using git config --global user.email email@example.com
3. Make sure that the git commit email is configured in your GitHub account settings, see https://github.com/settings/emails