threading macro instead of modify* macros

[Csepp <raingloom@riseup.net>]

Here is what I'm trying to do:
I have a package that inherits from the ocaml package.  Because of
Various Reasons (tm) I am using substitute-keyword-arguments and
modify-phases and I need to duplicate the shebang patching phase after
multiple phases.
What I wanted to do was write a helper function like:
(duplicate-after 'conf-2 'patch-sh 'patch-sh-again)
Then I realized I'd have to patch modify-phases to add this to its
grammar.
Then came the realizations that in Haskell this would instead be a
composition of curried applications, that's the thing that the cut macro
emulates in Scheme.
So, (modify-phases foo (add-after 'a 'b f) (delete 'c)) is really:
```
(compose
  (cut add-after <> 'a 'b f)
  (cut delete <> 'c))
```
And from having used Fennel a bit I know that it copied a very nice
syntactic feature from Clojure, called the threading macros, which
implements this exact idiom without having to write cut everywhere.

Guix currently has at least a few modify-whatever and
substitute-whatever macros that all hardcode a few operations.  Couldn't
they be made much cleaner and more general by using a solid library of
data manipulation functions and threading macros?
The phases list could be treated like the actual data type it is, an
ordered key-value mapping.

ps.: On that note, Guile has generics, why doesn't any code use it?
When I have to write things like string<? or package-native-inputs --
instead of < and native-inputs -- I feel like I'm in C again.