Add unit tests for kryo serialization of toIterableExecution

[tchow-stripe]

an older version of kryo (2.21) fails to serialize results of toIterableExecution. Issue: https://github.com/EsotericSoftware/kryo/issues/15

Caused by: com.esotericsoftware.kryo.KryoException: java.lang.IllegalArgumentException: Can not set final com.twitter.scalding.typed.TypedPipe field com.twitter.scalding.Execution$$anonfun$toIterable$1.t$8 to com.twitter.scalding.Execution$ToWrite$ToIterable
Serialization trace:
t$8 (com.twitter.scalding.Execution$$anonfun$toIterable$1)
result (com.twitter.scalding.Execution$WriteExecution)
$outer (com.twitter.scalding.Execution$WriteExecution$$anonfun$map$1)
result (com.twitter.scalding.Execution$WriteExecution)
prev (com.twitter.scalding.Execution$FlatMapped)
usedSketch (com.stripe.zoolander.util.features.PrecomputedSketched$SketchInputs)
si$1 (com.stripe.zoolander.util.features.PrecomputedSketched$SketchedPipe$$anonfun$1)
fn$1 (com.stripe.zoolander.util.features.PrecomputedSketched$SketchedPipe$$anonfun$flatMapWithReplicas$1$1)
$outer (com.stripe.zoolander.util.features.PrecomputedSketched$SketchedPipe$$anonfun$flatMapWithReplicas$1$1$$anonfun$apply$2)
next (com.twitter.scalding.typed.functions.FlatMapFunctions$FromMapCompose)
next (com.twitter.scalding.typed.functions.FlatMapFunctions$FromFlatMapCompose)
com$twitter$scalding$typed$functions$FlatMappedFn$$toFn (com.twitter.scalding.typed.functions.FlatMappedFn$Series)
    at com.esotericsoftware.kryo.serializers.FieldSerializer$ObjectField.read(FieldSerializer.java:626)
    at com.esotericsoftware.kryo.serializers.FieldSerializer.read(FieldSerializer.java:221)
    at com.esotericsoftware.kryo.Kryo.readObject(Kryo.java:648)

• added tests around the externalizer and KryoHadoop to assert on serialization. This test fails running against kryo 2.21 and succeeds in the development branch.
• make TypedPipe transient inside of ToWrite
• add default kryo serialization strategy for Execution, TypedPipe, Execution.ToWrite

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