Deprecate the nu_scripts version of stdlib-candidate (#1042)

Deprecates the existing `stdlib-candidate` directories in `nu_scripts`
since `std-rfc` has now transitioned to the main repo. Updates readme
and renamed directories.

stdlib-candidate-archive/stdlib-candidate-older/std-rfc/flatter.nu

@@ -0,0 +1,11 @@
+# A command to flatten many levels by using recursion
+# by @jturner 2/10/21
+# Example: sys | flatter 3
+def flatter [levels:int] {
+  let input = $in
+  if $levels > 0 {
+      $input | columns | reduce -f $input {|it acc| $acc | flatten $it } | flatter ($levels - 1)
+  } else {
+      $input
+  }
+}