Use 'mapm' instead of 'sequence' + 'map'.

Previously we'd use the (sequence M (map P L)) idiom just because 'mapm'
was slower (not specialized for the given monad).  This is no longer the
case since commit dcb95c1fc9.

* guix/gexp.scm (lower-inputs): Use (mapm M P L) instead of (sequence
M (map P L)).
(lower-references, gexp->sexp, imported-files): Likewise.
* guix/profiles.scm (profile-derivation): Likewise.
* guix/scripts/environment.scm (inputs->requisites): Likewise.
* guix/scripts/system.scm (copy-closure): Likewise.

guix/profiles.scm

@@ -1383,10 +1383,10 @@ are cross-built for TARGET."
                                                          #:target target)))
                        (extras (if (null? (manifest-entries manifest))
                                    (return '())
-                                   (sequence %store-monad
-                                             (map (lambda (hook)
-                                                    (hook manifest))
-                                                  hooks)))))
+                                   (mapm %store-monad
+                                         (lambda (hook)
+                                           (hook manifest))
+                                         hooks))))
     (define inputs
       (append (filter-map (lambda (drv)
                             (and (derivation? drv)