Skip data deconstruction for :db/ident

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If a seqable value is passed as a :db/ident value, then this is currently deconstructed into a linked list. Skip this operation for the :db/ident property only.

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The constructed triples appear this way:

cljs.user=> (def tx (d/transact conn {:tx-data [{:db/ident ["liz" 5] :name "Elizabeth"}]}))
#'cljs.user/tx
cljs.user=> (pprint (:tx-data @tx))
([:tg/node-5 :tg/entity true 3 true]
 [:tg/node-5 :db/ident :tg/node-6 3 true]
 [:tg/node-6 :tg/first "liz" 3 true]
 [:tg/node-6 :tg/rest :tg/node-7 3 true]
 [:tg/node-7 :tg/first 5 3 true]
 [:tg/node-6 :tg/contains "liz" 3 true]
 [:tg/node-6 :tg/contains 5 3 true]
 [:tg/node-5 :name "Elizabeth" 3 true])

Instead, what we want here is the following:

([:tg/node-5 :tg/entity true 3 true]
 [:tg/node-5 :db/ident ["liz" 3] 3 true]
 [:tg/node-5 :name "Elizabeth" 3 true])

This happens in zuko.entity.writer/property-vals. This function is called for every attribute/value pair, and generates the following:

1. If the value is a set, then that means that the attribute is reused for multiple values. For each value in the set, the following steps will occur for each attribute/value. (The implementor can ignore this).
2. If the value is a simple scalar, then the value will be that scalar.
3. If the value represent a structure, then allocated a node to represent the structure, and create a sequence of triples that use that node to represent the structure. The value will now be this node.
4. Cons the above sequence a single triple of the form [ entity-id attribute value ]

Steps 2 and 3 occur are performed by the function value-triples. The return value from this function is nested via an if-let due to defensive programming (I think an interface changed at some point), but in fact the value-triples function can never return nil. The if-let should probably be updated to a simple let.