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/renderverbatim.nim

@@ -107,6 +107,7 @@ proc extractRunnableExamplesSource*(conf: ConfigRef; n: PNode): string =
   var indent = info.col
   let numLines = numLines(conf, info.fileIndex).uint16
   var lastNonemptyPos = 0
+  result = ""
 
   var ldata = LineData(lineFirst: first.line.int, conf: conf)
   visitMultilineStrings(ldata, n[^1])