Also accept numeric strings when numbers are needed

[PhilippImhof]

Fixes #201

PHP typically uses "type juggling" to dynamically convert types according to the context, e. g. "12" / "2" is perfectly acceptable. Even more, the result will be of type int.

This PR changes the evaluator's behaviour to follow PHP's principle and accept numeric strings in all places where numbers are expected.

This has one side-effect: as PHP uses a special string concatenation operator (.) whereas we use + for this purpose, a choice has to be made w.r.t. priority. The decision is that the + operator is basically trying to add things and only falls back to mean concatenation if both arguments are non-numeric strings. If the user really wants to concatenate "1" and "2", they must use the join() function instead.

[coveralls]

Pull Request Test Coverage Report for Build 14819149286

Details

• 6 of 6 (100.0%) changed or added relevant lines in 3 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.001%) to 95.944%

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Totals
Change from base Build 14747761998:	0.001%
Covered Lines:	4021
Relevant Lines:	4191

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💛 - Coveralls