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

compiler/dfa.nim

@@ -715,10 +715,8 @@ proc genCall(c: var Con; n: PNode) =
   for i in 1..<n.len:
     gen(c, n[i])
     when false:
-      if t != nil and i < t.len and t[i].kind == tyVar:
-        # This is wrong! Pass by var is a 'might def', not a 'must def'
-        # like the other defs we emit. This is not good enough for a move
-        # optimizer.
+      if t != nil and i < t.len and t[i].kind == tyOut:
+        # Pass by 'out' is a 'must def'. Good enough for a move optimizer.
         genDef(c, n[i])
   # every call can potentially raise:
   if c.inTryStmt > 0 and canRaiseConservative(n[0]):