Allow serialization/deserialization of `Pair` to/from JSON array

[jdidion]

In WDL 1.1, serialization/deserializaton of Pair is not supported.

Since JSON arrays are allowed to be heterogeneous, we should support serialization of Pair[X, Y] to [X, Y] (as long as X and Y are both serializable) and deserialization of a heterogeneous two-element array [X, Y] to Pair[X, Y]. A homogeneous two-element array [X, X] would still deserialize to Array[X] and would require coercion to Pair[X, X].

This would also enable serialization of Map[X, Y] to an array of arrays, for X types other than String.

This change will affect the following areas of the spec:

• write_json will support serialization of Pairs and Maps of any type
• read_json will support deserialization of a heterogeneous two-element arrays to a Pair, and of an array of heterogeneous two-element arrays to a Map
• Coercion of Array[X] to Pair[X, X] will be supported for an array of exactly two elements
• The standard input/output JSON format will use array as the serialization format for a Pair, and array-of-arrays as the serialization format for a Map with non-String keys

[patmagee]

Overall I am supportive of this idea, although I think the expectation has been a pair would be serialized into something that looks like: { "left": ... , "right": }

I wonder how common the situation is where someone would want a heterogenous pair

[jdidion]

That is an option.

I like using an array because it doesn't require us to introduce a new custom format. Trying to deserialize a heterogeneous array into anything other than a Pair will fail. You could deserialize a homogeneous Pair into an array, but you could also deserialize {"left": X, "right": X} to a Map. There's no unambiguous serialization format for a Pair so in that case I'd argue to just go for the simplest solution.

[patmagee]

we should determine what the current engines do in this scenario, do they fail or do they have some sort of intermediary type conversion