read-print: Be more mindful of horizontal space in packages.

Perhaps due to their staged nature, packages tend to be ‘lopsided’: deeply
nested, with the most elaborate code on the {build,right-hand-,in}side, in
snippets and phases.

When we indent outer forms too eagerly, we can easily run short on
columns by the time we get to the build code, reducing readability.  A
few strategically-placed newlines early on can make a big difference.

* guix/read-print.scm (%newline-forms): Add a newline after opening a
<package> source field or an argument list.  Compensate by removing the
base32 special case, which is now unnecessary.

guix/read-print.scm

@@ -359,10 +359,11 @@ (define %newline-forms
   ;; the context in which they must appear.  This is similar to a special form
   ;; of 1, except that indent is 1 instead of 2 columns.
   (vhashq
-   ('arguments '(package))
+   ('source '(package))
-   ('sha256 '(origin source package))
-   ('base32 '(sha256 origin))
    ('git-reference '(uri origin source))
+   ('sha256 '(origin source package))
+   ('arguments '(package))
+   ('list '(arguments package))
    ('search-paths '(package))
    ('native-search-paths '(package))
    ('search-path-specification '())