math_parser: delete the shim

[andrei8l]

Summary

None

Purpose of change

It's time to delete the math_parser shim

Describe the solution

Simplify the functions that needed the shim, as much as possible, then delete the shim.

This also makes the u_val functions only available through math. They were still used through { "u_val": "xxx" } only in a couple of places and I've been meaning to disable that anyway, or at least decouple it from variable access.

Also corrected a few copy-paste, scoping, and truncation errors in the u_val code.

Describe alternatives you've considered

Reverting math: we're in too deep now.

Testing

Existing test units, for the most part. Some parts of u_val were probably never tested, given the errors in the code. I left a note about it in the documentation and will help port and/or fix them if they find use.

Additional context

I'm happy to say that the temporary shim did not turn out to be a permanent solution.

This can be reviewed by commit.

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[andrei8l]

Thanks for merging!