[5.6] RenameIndex support for migrations

[slava-vishnyakov]

As described here: laravel/ideas#443 and as suggested by @tomschlick here, opening a PR for migration call to rename index (->renameIndex('from', 'to')).

Additionally tested on local installations of MySQL, Postgres, MS SQL - those support renaming indexes.

SQLite doesn't support renaming indexes (as is correctly pointed in laravel/ideas#443), so I'm using Doctrine and its getRenameIndexSQL, unfortunately it's protected, so basically, I've copied the implementation (this is from Doctrine's source):

protected function getRenameIndexSQL($oldIndexName, Index $index, $tableName)
{
    return [
        $this->getDropIndexSQL($oldIndexName, $tableName),
        $this->getCreateIndexSQL($index, $tableName)
    ];
}

So, for SQLite it's drop and re-create (and requires Doctrine), for others - it's native calls.

(Edit: wrong code snippet)