Add a default for `UUID` primary keys

[helje5]

Let's say we have:

CREATE TABLE talent (
  talent_id UUID NOT NULL,
  name      TEXT NOT NULL
);

which generates

struct Talent {
  var id   : UUID
  var name : String
  init(id: UUID, name: String) { ... }
}

it would be nice if the init would be init(id: UUID = UUID(), name: String). As that kinda is the sane thing to do for new records.

[helje5]

Implemented.