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/std/stackframes.nim

@@ -4,7 +4,7 @@ const NimStackTraceMsgs = compileOption("stacktraceMsgs")
 template procName*(): string =
   ## returns current C/C++ function name
   when defined(c) or defined(cpp):
-    var name {.inject.}: cstring
+    var name {.inject, noinit.}: cstring
     {.emit: "`name` = __func__;".}
     $name
 
@@ -12,7 +12,7 @@ template getPFrame*(): PFrame =
   ## avoids a function call (unlike `getFrame()`)
   block:
     when NimStackTrace:
-      var framePtr {.inject.}: PFrame
+      var framePtr {.inject, noinit.}: PFrame
       {.emit: "`framePtr` = &FR_;".}
       framePtr
 
@@ -21,7 +21,7 @@ template setFrameMsg*(msg: string, prefix = " ") =
   ## in a given PFrame. Noop unless passing --stacktraceMsgs and --stacktrace
   when NimStackTrace and NimStackTraceMsgs:
     block:
-      var fr {.inject.}: PFrame
+      var fr {.inject, noinit.}: PFrame
       {.emit: "`fr` = &FR_;".}
       # consider setting a custom upper limit on size (analog to stack overflow)
       frameMsgBuf.setLen fr.frameMsgLen