[SPARK-14060][SQL] Move StringToColumn implicit class into SQLImplicits

## What changes were proposed in this pull request?
This patch moves StringToColumn implicit class into SQLImplicits. This was kept in SQLContext.implicits object for binary backward compatibility, in the Spark 1.x series. It makes more sense for this API to be in SQLImplicits since that's the single class that defines all the SQL implicits.

## How was this patch tested?
Should be covered by existing unit tests.

Author: Reynold Xin <rxin@databricks.com>
Author: Wenchen Fan <wenchen@databricks.com>

Closes #11878 from rxin/SPARK-14060.