[collection] explicitly make BoolList abstract interface (#875)

It couldn't be extended in practice since it doesn't have a
public constructor

Also made the only implementations final

Author: kevmoo

pkgs/collection/CHANGELOG.md

@@ -8,6 +8,7 @@
 - Optimize equality and hash code for maps by using `update` and a `values`
   iterator to avoid extra lookups.
 - Add `PriorityQueue.of` constructor and optimize adding many elements.
+- Address diagnostics from `strict_top_level_inference`.
 
 ## 1.19.1