Change operator precedence to C++ standard, string literals escaping too

[ped7g]

• add more tests to cover current system even for more complex expression (bugfix v1.x if there's still some bug)
• write tests for C++ precedence expression evaluator
• implement the new precedence, probably just patch the current code
• consider full refactoring to avoid the recursive approach, but research first what can be gain by avoiding the recursive evaluator, feels like the performance is suffering from it, but without benchmarking and research this is bogus claim.

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• string literals: reconsider to have the same escaping as C++, that means also in single apostrophes, maybe even support for full C++ escaping rules (maybe even those unfortunate octals? But only in strings?)
• reconsider having octals as C++ ... (I mean no, just no, it's horrible) (but let's consider it one more time for v2.x)

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help wanted: to add tests for more tricky expressions and exercising current precedence, to verify v1.x correctness