Add `to_model` generator

[Crisfole]

From here: apsislabs/slayer#50

It should be easy to write a class method that you can call to generate a to_<model> method.

[wkirby]

I'm open to this, but I want to sniff out exactly the use case here. I see two ways this can go: either in the form definition, you declare a model that this Form can be translated into:

class PersonForm < Slayer::Form
  models Person
  attribute :name, String
end

form = PersonForm.new(name: 'Joe')
form.to_model # => essentially: Person.new(name: 'Joe')

Or, we could use some magic with method_missing to try to inflect the class you intend to create.

class PersonForm < Slayer::Form
  attribute :name, String
end

form = PersonForm.new(name: 'Joe')
form.to_person # => inflect Person from `to_person`
form.to_contact # => inflect Contact f
form.to_car_dealership # => inflect CarDealership from `to_person`

# The above could all shorthand to a call to a #to_model method, like below
form.to_model(Namespaced::Person) # => bypass inflection, attempt to convert to Namespaced::Person

The second version here is more flexible, since it lets us convert a Form into any model, and any model that has a name attribute will get the value from the Form, but it also is more "magic," and as such inherently less trustworthy