services: Export 'service-extension' procedures.

* gnu/services.scm: Export service-extension-target and
service-extension-compute.  This allows for greater extensiblity of services
by enabling service extensions to be wrapped.  For example, the parameters
passed to the compute function can be modified, or the return value of the
compute function can be modified.

Signed-off-by: Ludovic Courtès <ludo@gnu.org>
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Christopher Baines 2017-01-15 09:00:59 +00:00 committed by Ludovic Courtès
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commit 7d8b59139a
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@ -38,6 +38,8 @@
#:use-module (ice-9 match)
#:export (service-extension
service-extension?
service-extension-target
service-extension-compute
service-type
service-type?