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/semobjconstr.nim

@@ -249,7 +249,7 @@ proc semConstructFields(c: PContext, recNode: PNode,
           else:
             badDiscriminatorError()
         elif discriminatorVal.sym.kind notin {skLet, skParam} or
-            discriminatorVal.sym.typ.kind == tyVar:
+            discriminatorVal.sym.typ.kind in {tyVar}:
           localError(c.config, discriminatorVal.info,
             "runtime discriminator must be immutable if branch fields are " &
             "initialized, a 'let' binding is required.")