Solve problems of sharing + for addition and string concat

DaphneDSL uses operator `+` for both (elementwise) addition and (elementwise) string concatenation. However, at parse-time we cannot safely know which of the two operations we need to create, because argument types might still be unknown in some situations. Our initial attempt was to create a `EwAddOp` during parsing and to rewrite it during canonicalization if arguments turn out to be strings. Nevertheless, this causes problems, since addition is commutative and string concat ist not: the compiler might have swapped operands by then, leading to wrong string concat results.

As a quick fix, we remove the `Commutative` trait from `EwAddOp`. However, we need a clean solution.

Begin with EwAddOp and its Commutative trait, then trace its parse-time creation and canonicalization rewrite. Done means a clean representation or rewrite strategy distinguishes elementwise addition from string concatenation without allowing operand reordering to change concatenation results.

Written by the indexing model from the issue text.