Adds [Heap]PriorityQueue.of constructor. (#734)

Introduces efficient (linear-number of comparisons) "heapify" algorithm for converting an unsorted list to a heap-sorted list, using it for the of constructor, and after a large addAll operation, when it's presumed faster than just bubbling down all the new elements.

Also rewrites HeapPriorityQueue to use a growable list as backing array, instead of implementing the same thing using the double-when-full algorithm, and still having to deal with nullable cells. The platform growable list implementation is assumed to efficiently avoid some of those null checks.

Author: lrhn

pkgs/collection/CHANGELOG.md

@@ -5,6 +5,10 @@
   `Map.entries`.
 - Explicitly mark `BoolList` as `abstract interface`
 - Address diagnostics from `strict_top_level_inference`.
+- 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