Support concrete playback for arrays of length 65 or greater

[carolynzech]

Problem

When CBMC generates a JSON trace for a nondeterministic array, it will output a key-value pair elements: [array], where array contains concrete values for each element in the array. If the array is length 64 or shorter, it will also output the elements of the array as separate values in the trace (so each element is in the trace twice). Prior to this PR, concrete playback relied on the latter output format to find elements of an array. However, when the array was length 65 or greater, those elements wouldn't be values in their own right, so we wouldn't find any values for the array and just output an empty array.

Solution

This PR changes our representation of concrete values to handle arrays explicitly; i.e., to look for the elements array and populate the concrete values of the array from that instead.

Callouts

For those wondering why the playback_already_existed test changed, it's because we're hashing ConcreteItem instead of ConcreteValue, so the hash changes.

Resolves #3787

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 and MIT licenses.

[carolynzech]

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