(*features*): Remove array and array-for-each, core

(*features*): Remove array and array-for-each, core
definitions are insufficient for latest slib.
(t, nil): New constants slib says are supposed to exist.
(call-with-open-ports, browse-url): New functions for latest slib.
Implementations taken from Template.scm (public domain).
(open-file): Extend core definition to accept symbols for the mode,
required by latest slib.
(delete-file): Replace core definition with version returning #t/#f as
per slib spec.
(system): Mark as #:replace to suppress override warning, use new
style "(@ (guile) system)" to call core function.