[SPARK-11116][SQL] First Draft of Dataset API

*This PR adds a new experimental API to Spark, tentitively named Datasets.*

A `Dataset` is a strongly-typed collection of objects that can be transformed in parallel using functional or relational operations.  Example usage is as follows:

### Functional
```scala
> val ds: Dataset[Int] = Seq(1, 2, 3).toDS()
> ds.filter(_ % 1 == 0).collect()
res1: Array[Int] = Array(1, 2, 3)
```

### Relational
```scala
scala> ds.toDF().show()
+-----+
|value|
+-----+
|    1|
|    2|
|    3|
+-----+

> ds.select(expr("value + 1").as[Int]).collect()
res11: Array[Int] = Array(2, 3, 4)
```

## Comparison to RDDs
 A `Dataset` differs from an `RDD` in the following ways:
  - The creation of a `Dataset` requires the presence of an explicit `Encoder` that can be
    used to serialize the object into a binary format.  Encoders are also capable of mapping the
    schema of a given object to the Spark SQL type system.  In contrast, RDDs rely on runtime
    reflection based serialization.
  - Internally, a `Dataset` is represented by a Catalyst logical plan and the data is stored
    in the encoded form.  This representation allows for additional logical operations and
    enables many operations (sorting, shuffling, etc.) to be performed without deserializing to
    an object.

A `Dataset` can be converted to an `RDD` by calling the `.rdd` method.

## Comparison to DataFrames

A `Dataset` can be thought of as a specialized DataFrame, where the elements map to a specific
JVM object type, instead of to a generic `Row` container. A DataFrame can be transformed into
specific Dataset by calling `df.as[ElementType]`.  Similarly you can transform a strongly-typed
`Dataset` to a generic DataFrame by calling `ds.toDF()`.

## Implementation Status and TODOs

This is a rough cut at the least controversial parts of the API.  The primary purpose here is to get something committed so that we can better parallelize further work and get early feedback on the API.  The following is being deferred to future PRs:
 - Joins and Aggregations (prototype here https://github.com/apache/spark/commit/f11f91e6f08c8cf389b8388b626cd29eec32d937)
 - Support for Java

Additionally, the responsibility for binding an encoder to a given schema is currently done in a fairly ad-hoc fashion.  This is an internal detail, and what we are doing today works for the cases we care about.  However, as we add more APIs we'll probably need to do this in a more principled way (i.e. separate resolution from binding as we do in DataFrames).

## COMPATIBILITY NOTE
Long term we plan to make `DataFrame` extend `Dataset[Row]`.  However,
making this change to che class hierarchy would break the function signatures for the existing
function operations (map, flatMap, etc).  As such, this class should be considered a preview
of the final API.  Changes will be made to the interface after Spark 1.6.

Author: Michael Armbrust <michael@databricks.com>

Closes #9190 from marmbrus/dataset-infra.

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