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/pure/bitops.nim

@@ -395,6 +395,8 @@ proc firstSetBitNim(x: uint64): int {.inline, noSideEffect.} =
   if k == 0:
     k = uint32(v shr 32'u32) and 0xFFFFFFFF'u32
     result = 32
+  else:
+    result = 0
   result += firstSetBitNim(k)
 
 proc fastlog2Nim(x: uint32): int {.inline, noSideEffect.} =