Try to prune the Gnus registry if it's full.

Try to prune the Gnus registry if it's full.

* gnus-registry.el (gnus-registry-insert): Add wrapper that calls
`registry-prune' if `registry-full' returns t.
(gnus-registry-handle-action)
(gnus-registry-get-or-make-entry, gnus-registry-set-id-key)
(gnus-registry-usage-test): Use it.

* registry.el (registry-full): Add convenience method.
(registry-insert): Use it.