Composite primary key migration

[Whaxion]

This PR is about the migration part of the composite primary key implementation.
It adds composite_primary_key to the create table migrator.
It works like this

def migrate
  create table_for(UserPost) do
    add_belongs_to user : User, on_delete: :cascade
    add_belongs_to post : Post, on_delete: :cascade
    primary_key :user_id, :post_id
  end
end

There is still some work to do with add_belongs_to if it's a composite PK but for that it needs some work on the model part.
Issue #129

[matthewmcgarvey]

Can you explain why you added this constraints array instead of adding it to the rows array?

[Whaxion]

It's because constraints must be at the end of create table declaration, so if it was in row, the user should call composite_primary_key at the end of the migration.
So user couldn't do a migration like this:

def migrate
  create table_for(UserPost) do
    add_belongs_to user : User, on_delete: :cascade
    add_belongs_to post : Post, on_delete: :cascade
    composite_primary_key :user_id, :post_id

    add example : String
  end
end

But should do this:

def migrate
  create table_for(UserPost) do
    add_belongs_to user : User, on_delete: :cascade
    add_belongs_to post : Post, on_delete: :cascade

    add example : String

    composite_primary_key :user_id, :post_id
  end
end

And I find that's sad that we can't put the composite_primary_key where we want so I added a constraints array.
But if that's not okay I can change that.

[matthewmcgarvey]

@Whaxion I didn't know that it had to be at the end. Good thinking. Could you add a spec showing that a composite key is added at the end of the sql statement even if it is declared first? I'd like to make sure we remember to check for that behavior even if we change things in the future.

[matthewmcgarvey]

Nice job @Whaxion 🎉