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fix: lazier evaluation of naryop and proper symbolic string concat

Previously, to evaluate a naryop we started by checking if the list of
expressions contained only values. If so, we would perform another
traversal over the list to extract the actual values.
In the happy path this would cause two traversals over the list of
expressions.
To fix this we can iterate through the list and extract the values until
either one of two things occurs:

1. We reach the end of the list -> dispatch concrete evaluation
2. We get something other than a value -> stop

Doing this allows us to preserve the short-circuit evaluation of
naryops with symbolic arguments and also reduce a traversal during the
evaluation of concrete naryops.

This commit also addresses the previous incorrect simplification of the
operation `str.++` (string concatenation). Previously, we only joined
daisy-chained concatenation operators if the lists contained only two
arguments. This is wrong because the operator can take a list.
This resulted in expressions not being simplified when the operator was
used with 3 or more arguments.

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