Prefer APersistentMap over IPersistentMap

[weavejester]

The IPersistentMap interface is implemented by records as well as normal map structures, so when serializing records Nippy incorrectly encodes them as maps.

user=> (require '[taoensso.nippy :as nippy])
nil
user=> (defrecord Foo [x])
user.Foo
user=> (nippy/thaw (nippy/freeze (Foo. 1)))
{:x 1}

The APersistentMap abstract class, on the other hand, is implemented by all map types except for records:

user=> (instance? clojure.lang.APersistentMap (hash-map))
true
user=> (instance? clojure.lang.APersistentMap (array-map))
true
user=> (instance? clojure.lang.APersistentMap (sorted-map))
true
user=> (instance? clojure.lang.APersistentMap (Foo. 1))
false

For this reason it's better for Nippy to dispatch its freezer off of APersistentMap instead of IPersistentMap.