Refactor anonymisable interface into fields and destroy groups

[nickcampbell18]

Since it's incompatible to have both destroy and field-level configuration, this PR modifies a fairly hefty chunk of the anony internals to accomplish what felt like a reasonable goal: isolating the field-level strategies and the destroy strategies into different classes. The code is structured so that it's impossible to define both of these, because the configuration is essentially an enum on a single instance variable (Anony::ModelConfig#@strategy).

This makes the public interface a bit more unwieldy, but I'd argue this makes sense in the context of some proposed changes in discussion. An example will hopefully be illustrative:

class Manager
  anonymise do
    fields do
      hex :first_name, :last_name
      ignore :company_id
    end

    skip_if { some_condition? }

    before { puts "Starting to anonymise..." }
  end
end

As you can see, hex and ignore don't logically belong at the same level as skip_if or before/after. This isolates them into a fields block, which is encapsulated as an Anony::Strategies::Fields instance (hence the first commit moving other things around). Sandi Metz would be proud of the subsequent approach: we delegate the valid?, validate! and apply methods to those strategies.

[nickcampbell18]

I'm incredibly happy about this part.

[matt-thomson]

And so you should be.

[matt-thomson]

Like it very much. Agree that moving over to this is a slight pain, but should be pretty mechanical.

We don't have to tackle it now, but I'm wondering if there's a use case for specifying your own "model-level" strategies?

[matt-thomson]

And so you should be.

[matt-thomson]

Couple of minor comments, otherwise this looks good 👍