[SPARK-9142][SQL] Removing unnecessary self types in expressions.

Also added documentation to expressions to explain the important traits and abstract classes.

Author: Reynold Xin <rxin@databricks.com>

Closes #7550 from rxin/remove-self-types and squashes the following commits:

b2a3ec1 [Reynold Xin] [SPARK-9142][SQL] Removing unnecessary self types in expressions.

sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/ExpectsInputTypes.scala

@@ -29,7 +29,7 @@ import org.apache.spark.sql.catalyst.analysis.HiveTypeCoercion.ImplicitTypeCasts
  *
  * Most function expressions (e.g. [[Substring]] should extends [[ImplicitCastInputTypes]]) instead.
  */
-trait ExpectsInputTypes { self: Expression =>
+trait ExpectsInputTypes extends Expression {
 
   /**
    * Expected input types from child expressions. The i-th position in the returned seq indicates
@@ -60,6 +60,6 @@ trait ExpectsInputTypes { self: Expression =>
 /**
  * A mixin for the analyzer to perform implicit type casting using [[ImplicitTypeCasts]].
  */
-trait ImplicitCastInputTypes extends ExpectsInputTypes { self: Expression =>
+trait ImplicitCastInputTypes extends ExpectsInputTypes {
   // No other methods
 }

sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/Expression.scala

@@ -19,19 +19,12 @@ package org.apache.spark.sql.catalyst.expressions
 
 import org.apache.spark.sql.catalyst.InternalRow
 import org.apache.spark.sql.catalyst.analysis.{TypeCheckResult, UnresolvedAttribute}
-import org.apache.spark.sql.catalyst.expressions.codegen.{CodeGenContext, GeneratedExpressionCode}
+import org.apache.spark.sql.catalyst.expressions.codegen._
 import org.apache.spark.sql.catalyst.trees.TreeNode
 import org.apache.spark.sql.types._
 
 ////////////////////////////////////////////////////////////////////////////////////////////////////
-// This file defines the basic expression abstract classes in Catalyst, including:
-// Expression: the base expression abstract class
-// LeafExpression
-// UnaryExpression
-// BinaryExpression
-// BinaryOperator
-//
-// For details, see their classdocs.
+// This file defines the basic expression abstract classes in Catalyst.
 ////////////////////////////////////////////////////////////////////////////////////////////////////
 
 /**
@@ -39,9 +32,21 @@ import org.apache.spark.sql.types._
  *
  * If an expression wants to be exposed in the function registry (so users can call it with
  * "name(arguments...)", the concrete implementation must be a case class whose constructor
- * arguments are all Expressions types.
+ * arguments are all Expressions types. See [[Substring]] for an example.
+ *
+ * There are a few important traits:
+ *
+ * - [[Nondeterministic]]: an expression that is not deterministic.
+ * - [[Unevaluable]]: an expression that is not supposed to be evaluated.
+ * - [[CodegenFallback]]: an expression that does not have code gen implemented and falls back to
+ *                        interpreted mode.
+ *
+ * - [[LeafExpression]]: an expression that has no child.
+ * - [[UnaryExpression]]: an expression that has one child.
+ * - [[BinaryExpression]]: an expression that has two children.
+ * - [[BinaryOperator]]: a special case of [[BinaryExpression]] that requires two children to have
+ *                       the same output data type.
  *
- * See [[Substring]] for an example.
  */
 abstract class Expression extends TreeNode[Expression] {
 
@@ -176,7 +181,7 @@ abstract class Expression extends TreeNode[Expression] {
  * An expression that cannot be evaluated. Some expressions don't live past analysis or optimization
  * time (e.g. Star). This trait is used by those expressions.
  */
-trait Unevaluable { self: Expression =>
+trait Unevaluable extends Expression {
 
   override def eval(input: InternalRow = null): Any =
     throw new UnsupportedOperationException(s"Cannot evaluate expression: $this")
@@ -185,11 +190,11 @@ trait Unevaluable { self: Expression =>
     throw new UnsupportedOperationException(s"Cannot evaluate expression: $this")
 }
 
+
 /**
  * An expression that is nondeterministic.
  */
-trait Nondeterministic { self: Expression =>
-
+trait Nondeterministic extends Expression {
   override def deterministic: Boolean = false
 }
 

sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/codegen/CodegenFallback.scala

@@ -22,7 +22,7 @@ import org.apache.spark.sql.catalyst.expressions.Expression
 /**
  * A trait that can be used to provide a fallback mode for expression code generation.
  */
-trait CodegenFallback { self: Expression =>
+trait CodegenFallback extends Expression {
 
   protected def genCode(ctx: CodeGenContext, ev: GeneratedExpressionCode): String = {
     ctx.references += this