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

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 dcb95c1fc936d74dfdf84b7e59eff66cb99c5a63.

* 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.