[SPARK-8117] [SQL] Push codegen implementation into each Expression

This PR move codegen implementation of expressions into Expression class itself, make it easy to manage.

It introduces two APIs in Expression:
```
def gen(ctx: CodeGenContext): GeneratedExpressionCode
def genCode(ctx: CodeGenContext, ev: GeneratedExpressionCode): Code
```

gen(ctx) will call genSource(ctx, ev) to generate Java source code for the current expression. A expression needs to override genSource().

Here are the types:
```
type Term String
type Code String

/**
 * Java source for evaluating an [[Expression]] given a [[Row]] of input.
 */
case class GeneratedExpressionCode(var code: Code,
                               nullTerm: Term,
                               primitiveTerm: Term,
                               objectTerm: Term)
/**
 * A context for codegen, which is used to bookkeeping the expressions those are not supported
 * by codegen, then they are evaluated directly. The unsupported expression is appended at the
 * end of `references`, the position of it is kept in the code, used to access and evaluate it.
 */
class CodeGenContext {
  /**
   * Holding all the expressions those do not support codegen, will be evaluated directly.
   */
  val references: Seq[Expression] = new mutable.ArrayBuffer[Expression]()
}
```

This is basically #6660, but fixed style violation and compilation failure.

Author: Davies Liu <davies@databricks.com>
Author: Reynold Xin <rxin@databricks.com>

Closes #6690 from rxin/codegen and squashes the following commits:

e1368c2 [Reynold Xin] Fixed tests.
73db80e [Reynold Xin] Fixed compilation failure.
19d6435 [Reynold Xin] Fixed style violation.
9adaeaf [Davies Liu] address comments
f42c732 [Davies Liu] improve coverage and tests
bad6828 [Davies Liu] address comments
e03edaa [Davies Liu] consts fold
86fac2c [Davies Liu] fix style
02262c9 [Davies Liu] address comments
b5d3617 [Davies Liu] Merge pull request #5 from rxin/codegen
48c454f [Reynold Xin] Some code gen update.
2344bc0 [Davies Liu] fix test
12ff88a [Davies Liu] fix build
c5fb514 [Davies Liu] rename
8c6d82d [Davies Liu] update docs
b145047 [Davies Liu] fix style
e57959d [Davies Liu] add type alias
3ff25f8 [Davies Liu] refactor
593d617 [Davies Liu] pushing codegen into Expression