Re: A friendlier API for operating-system declarations

[antlers <autumnalantlers@gmail.com>]

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Anecdotally, my personal configurations are built out of composable
operating-system transformers that add packages, services, etc. through
trees of inherited records.

That's fairly straightforward for eg. adding packages, but becomes more
complicated when you consider modifying service configurations (which you
might want to inherit from when present and create when absent), nested
service configurations (inheriting multiple times, the whole away down), or
modifying kernel-arguments (where you might want to strip values for the
option that you're setting from existing strings).

I feel that operating-system record fields should be `extendable` in the
same way as services, and have always assumed that that's basically what
RDE's `features` are.

It would be great to have structure that eliminates some of the nesting and
boilerplate of an expression like this:
```
;; imagine if this was (imperative-ness aside) as simple as:
;; lambda os: os.services(foo).z.foo += bar

(lambda (os)
  (operating-system
    (inherit os)
    (sevices
      (modify-services
        (operating-system-services os)
        (y-service-type
          config => (y-configuration
                      (inherit config)
                      (z-configuration
                        (z-configuration
                         (inherit (y-configuration-z config)
                         (foo (cons bar
                                    (z-configuration-foo
                                      (y-configuration-z config)))))))))))))

;; And if we want to create the service when it doesn't exist?

(lambda (os)
  (operating-system
    (inherit os)
    (sevices
      (if (find (lambda (s)
                  (eq (service-kind s) y))
                (operating-system-services os))
          (modify-services
            (operating-system-services os)
            (y-service-type
              config => (y-configuration
                          (inherit config)
                          (z-configuration
                            (z-configuration
                             (inherit (y-configuration-z config)
                             (foo (cons bar
                                        (z-configuration-foo
                                          (y-configuration-z
config))))))))))
          (cons (service y-service-type
                  (y-configuration
                    (z-configuration
                      (z-configuration
                        (foo (list bar))))))
                (operating-system-services os))))))
```

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