[SPARK-21196] Split codegen info of query plan into sequence

codegen info of query plan can be very long.
In debugging console / web page, it would be more readable if the subtrees and corresponding codegen are split into sequence.

Example:

```java
codegenStringSeq(sql("select 1").queryExecution.executedPlan)
```
The example will return Seq[(String, String)] of length 1, containing the subtree as string and the corresponding generated code.

The subtree as string:

> (*Project [1 AS 1#0]
> +- Scan OneRowRelation[]

The generated code:
```java
/* 001 */ public Object generate(Object[] references) {
/* 002 */   return new GeneratedIterator(references);
/* 003 */ }
/* 004 */
/* 005 */ final class GeneratedIterator extends org.apache.spark.sql.execution.BufferedRowIterator {
/* 006 */   private Object[] references;
/* 007 */   private scala.collection.Iterator[] inputs;
/* 008 */   private scala.collection.Iterator inputadapter_input;
/* 009 */   private UnsafeRow project_result;
/* 010 */   private org.apache.spark.sql.catalyst.expressions.codegen.BufferHolder project_holder;
/* 011 */   private org.apache.spark.sql.catalyst.expressions.codegen.UnsafeRowWriter project_rowWriter;
/* 012 */
/* 013 */   public GeneratedIterator(Object[] references) {
/* 014 */     this.references = references;
/* 015 */   }
/* 016 */
/* 017 */   public void init(int index, scala.collection.Iterator[] inputs) {
/* 018 */     partitionIndex = index;
/* 019 */     this.inputs = inputs;
/* 020 */     inputadapter_input = inputs[0];
/* 021 */     project_result = new UnsafeRow(1);
/* 022 */     project_holder = new org.apache.spark.sql.catalyst.expressions.codegen.BufferHolder(project_result, 0);
/* 023 */     project_rowWriter = new org.apache.spark.sql.catalyst.expressions.codegen.UnsafeRowWriter(project_holder, 1);
/* 024 */
/* 025 */   }
/* 026 */
/* 027 */   protected void processNext() throws java.io.IOException {
/* 028 */     while (inputadapter_input.hasNext() && !stopEarly()) {
/* 029 */       InternalRow inputadapter_row = (InternalRow) inputadapter_input.next();
/* 030 */       project_rowWriter.write(0, 1);
/* 031 */       append(project_result);
/* 032 */       if (shouldStop()) return;
/* 033 */     }
/* 034 */   }
/* 035 */
/* 036 */ }
```
## What changes were proposed in this pull request?
add method codegenToSeq: split codegen info of query plan into sequence

## How was this patch tested?
unit test

cloud-fan gatorsmile
Please review http://spark.apache.org/contributing.html before opening a pull request.

Author: Wang Gengliang <ltnwgl@gmail.com>

Closes #18409 from gengliangwang/codegen.