Add multiple inspectors

* NEWS.md: Describe recently added features.

* sly.el (sly-completing-read): New function.
(sly-complete-symbol-function): New variable.
(sly-read-package-name, sly-read-interactive-args)
(sldb-invoke-restart-by-name, sly-read-connection)
(sly-read-contrib-name): Use `sly-completing-read'
(sly-dispatch-event): Use `sly--open-inspector'
(sly-eval-for-inspector): Tweak and add `inspector-name' arg.
(sly-read-inspector-name, sly-maybe-read-inspector-name): New
function,
(sly--open-inspector): Renamed from
`sly-open-inspector'. Considers inspector names.
(sly-inspector-reinspect): Take optional argument `inspector-name'

* lib/lisp/swank.lisp (ensure-istate-metadata): Resurrected.
(inspect-object): Be sure to make the istate first and only then
set its content.
(emacs-inspect :around): Set bindings for inspecting here.
(eval-for-inspector): Only bind *INSPECTOR* if some name is given.