[SPARK-15459][SQL] Make Range logical and physical explain consistent

## What changes were proposed in this pull request?
This patch simplifies the implementation of Range operator and make the explain string consistent between logical plan and physical plan. To do this, I changed RangeExec to embed a Range logical plan in it.

Before this patch (note that the logical Range and physical Range actually output different information):
```
== Optimized Logical Plan ==
Range 0, 100, 2, 2, [id#8L]

== Physical Plan ==
*Range 0, 2, 2, 50, [id#8L]
```

After this patch:
If step size is 1:
```
== Optimized Logical Plan ==
Range(0, 100, splits=2)

== Physical Plan ==
*Range(0, 100, splits=2)
```

If step size is not 1:
```
== Optimized Logical Plan ==
Range (0, 100, step=2, splits=2)

== Physical Plan ==
*Range (0, 100, step=2, splits=2)
```

## How was this patch tested?
N/A

Author: Reynold Xin <rxin@databricks.com>

Closes #13239 from rxin/SPARK-15459.