Add sly-retro contrib and enable it by default

This contrib ensures that SLY is not locked out of the existing SWANK
communication protocol.

* sly-autoloads.el (sly-contribs): Add `sly-retro' contrib by default.

* sly.el (sly-contribs): Add `sly-retro' contrib by default.
(sly-net-send-translator): New variable.
(sly-net-send): Use it.

* slynk.asd (:slynk-retro): New defsystem.

* contrib/sly-retro.el: New contrib.

* contrib/slynk-retro.lisp (:slynk-retro): New package.

* lib/lisp/slynk-rpc.lisp (*translating-swank-to-slynk*): New variable.
(*validate-input*): Removed.
(read-form): Use *TRANSLATING-SWANK-TO-SLYNK* and TRANSLATING-READ.
(validating-read): Removed.
(simple-read): Removed.
(translating-read): New.
(maybe-convert-package-designator): New.

* lib/lisp/slynk.lisp (*slynk-require-hook*): New hook.
(slynk-require): Run *SLYNK-REQUIRE-HOOK*
NEWS.md tweaks