Let models have multiple direct associations with the same type

[gfx]

This patch introduces the Aliases class to the runtime library, which allocates table name aliases dynamically.

Resolve #189

---

In ModelWithDirectAssociation2, the $DEFAULT_RESULT_COLUMNS should be something like this:

  final String[] $DEFAULT_RESULT_COLUMNS = {
    "`m1`.`author`", // this.author.getSafeName()
      "`a2`.`note`", // this.author.associationSchema.note.getSafeName()
      "`a2`.`name`"
    ,
    "`m1`.`author2`", // this.author1.getSafeName()
      "`a3`.`note`", // this.author1.associationSchema.note.getSafeName()
      "`a3`.`name`"
    ,
    "`m1`.`publisher`",
      "`p4`.`name`",
      "`p4`.`started_year`",
      "`p4`.`started_month`",
      "`p4`.`id`"
    ,
    "`m1`.`publisher2`",
      "`p5`.`name`",
      "`p5`.`started_year`",
      "`p5`.`started_month`",
      "`p5`.`id`"
    ,
    "`m1`.`note`",
    "`m1`.`name`"
  };

JOINs:

SELECT * FROM ModelWithDirectAssociation2 AS m1
  LEFT OUTER JOIN Author AS a1 ON
    m1.id = a1.id
  -- ...