[SPARK-2218] rename Equals to EqualTo in Spark SQL expressions.

Due to the existence of scala.Equals, it is very error prone to name the expression Equals, especially because we use a lot of partial functions and pattern matching in the optimizer.

Note that this sits on top of #1144.

I think this can be simplified to:

```scala
assert(project.collect { case e: EqualsTo => e }.size === 1)
```