Friendlier interactive evaluation

* sly.el (sly-with-popup-buffer): Pass some useful args to
`pop-to-buffer' or `display-buffer'.
(sly-display-eval-result): Decide whether to describe in echo area
or separate buffer.
(sly-show-description): Description buffers might as well enter
`lisp-mode'.

* lib/lisp/swank.lisp (*echo-area-prefix*): Delete.
(format-values-for-emacs): Renamed and reworked. Multi-line
multiple values are not printed with commas in between.
(values-to-string, interactive-eval-region)
(eval-string-in-frame): Use FORMAT-VALUES-FOR-EMACS
More preparatory work for multiple inspectors.

* sly.el (sly-open-inspector): Accept `serial' key.

* lib/lisp/swank.lisp
(defstruct connection): Add inspectors slot.
(drop-unprocessed-events): Fix compilation warning.
(defclass inspector): New class.
(*inspector-verbose*): Remove variable.
(find-inspector): New helper.
(istate): Rename METADATA-PLIST to METADATA. Remove NEXT and
PREVIOUS. Add SERIAL.
(*inspector*): New dynamically bound variable.
(current-inspector, current-istate): New function.
(ensure-istate-metadata): Delete unused function.
(emacs-inspect/istate): Remove helper.
(reset-inspector, inspect-object, prepare-title)
(inspector-nth-part, inspector-range, inspector-call-nth-action)
(inspector-pop, inspector-next, inspector-reinspect)
(inspector-toggle-verbose, inspector-history)
(describe-inspectee): Reimplement for new inspector mechanics.
(eval-for-inspector): Bind *INSPECTOR*.