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/system/sets.nim

@@ -31,6 +31,7 @@ proc countBits64(n: uint64): int {.compilerproc, inline.} =
 
 proc cardSet(s: NimSet, len: int): int {.compilerproc, inline.} =
   var i = 0
+  result = 0
   when defined(x86) or defined(amd64):
     while i < len - 8:
       inc(result, countBits64((cast[ptr uint64](s[i].unsafeAddr))[]))