Add ability to generate a record of symbol proxies for large forms

[thomashoneyman]

What does this pull request do?

Adds a function mkSProxies which allows you to provide a proxy for your form type and receive a record filled with proxies for each of the fields inside. For small forms this won't provide much in the way of savings, but for larger forms it reduces the boilerplate required to create a form and ensures that the proxies stay in sync with the actual fields in the form.

Where should the reviewer start?

Review the new class in Formless.Spec.Transform and the changes in the examples, especially in Polyform and the real-world example.

Other Notes:

This does not require any changes to existing code as you are still free to define symbol proxies manually if you'd like. This is more of a preparatory improvement to the library's functionality to try and provide per-field validation & modification than a major feature addition.