Introduce a DSL to construct a chain

Most of the work is already done. Mutant can't find any mutations but shows quite some "noop fails" (most likely due to errors in `to_source`). Most of the devtools rake tasks are not yet happy, as this code is still missing docs and flay/flog/reek adaptions.

- [ ] Make `rake ci` happy
- [ ] Add common processors (at least `call`, maybe also `wrap` and `evaluate`)
- [ ] Add info to the README

The DSL allows you to build environments and construct chains like so:

```ruby
env = Substation::Environment.build do
  register :authenticate, Processors::Authenticator
  register :authorize,    Processors::Authorizer
  register :sanitize,     Substation::Stack::Sanitizer
  register :validate,     Substation::Stack::Validator
  register :call,         Substation::Stack::Pivot
  register :wrap,         Substation::Stack::Wrapper
end

AUTHENTICATED = env.chain { authenticate }
AUTHORIZED    = env.chain(AUTHENTICATED) { authorize }

CREATE_PERSON = env.chain(AUTHORIZED) do
  sanitize Sanitizers::NEW_PERSON
  validate Validators::NEW_PERSON

  call Actions::CreatePerson

  wrap Presenters::Person
  wrap Views::Person
end
```