Add support for nested records, such as Record2<R1, R2>

[lukaseder]

This could be useful for users trying to restore the table product created from JOIN expressions in the currently more generic Result<?> type. In other words, it should be possible to write:

Result<Record2<CountryRecord, CityRecord>> result = 
create.select()
      .from(COUNTRY)
      .leftOuterJoin(CITY).on(...)
      .fetch();

The concrete API implications would have to be well thought through. Probably, this should only work when explicitly selecting various row value expressions, e.g.

Result<Record2<CountryRecord, CityRecord>> result = 
create.select(row(COUNTRY), row(CITY))
      .from(COUNTRY)
      .leftOuterJoin(CITY).on(...)
      .fetch();

Note, this is natively supported in PostgreSQL, as can be seen in this blog post here:
http://blog.jooq.org/2014/04/15/how-to-do-this-with-sql-of-course/

  select
    d.*,
    row(sektor, row, scene1, scene2) block
  from d

or simply:

  select d
  from d

[lukaseder]

Potential duplicate of #3886

[lukaseder]

The embeddable type feature (#2530) proves to be a lot more difficult to implement than first expected. We'll postpone the feature, and features built on top of it, to 3.13.

[lukaseder]

This does seem to be a duplicate of #3886, where a Row[N] can be wrapped as a Field<Record[N]> using DSL::rowField