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/guards.nim

@@ -46,7 +46,7 @@ proc isLet(n: PNode): bool =
     if n.sym.kind in {skLet, skTemp, skForVar}:
       result = true
     elif n.sym.kind == skParam and skipTypes(n.sym.typ,
-                                             abstractInst).kind != tyVar:
+                                             abstractInst).kind notin {tyVar}:
       result = true
 
 proc isVar(n: PNode): bool =