v0.11.0: Plugin / Slot system

- defslot: register render functions into named slots with ordering
- slot-render: call all registered render-fns for a slot
- Slot modes designed (stack/replace/single-winner) but mode dispatch
  is implicit via the registration API
- slot-p, clear-slot, list-slots for lifecycle management
- Slots stored in a hash table keyed by string (equal test)
- 4 tests, 100% passing

src/components/slot-package.lisp

@@ -0,0 +1,9 @@
+(defpackage :cl-tty.slot
+  (:use :cl)
+  (:export
+   #:defslot
+   #:slot-render
+   #:slot-p
+   #:clear-slot
+   #:list-slots
+   #:*slots*))