Ability to mark fields as non-nilable

[vladfaust]

In real world models can have fields which are NOT NULL; but in Core developer still has to write model.field.not_nil! when working with such a field.

Proposal:

class User < Core::Model
  schema do
    field :username, String
    field :age, Int32?
  end
end

Should expand to:

class User < Core::Model
  getter! username, String?
  getter age : Int32?
end