init checks and 'out' parameters (#14521)

* I don't care about observable stores
* enforce explicit initializations
* cleaner code for the stdlib
* stdlib: use explicit initializations
* make tests green
* algorithm.nim: set result explicitly
* remove out parameters and bring the PR into a mergable state
* updated the changelog

lib/pure/algorithm.nim

@@ -187,6 +187,7 @@ proc binarySearch*[T, K](a: openArray[T], key: K,
     else:
       return -1
 
+  result = 0
   if (len and (len - 1)) == 0:
     # when `len` is a power of 2, a faster shr can be used.
     var step = len shr 1