Adds Modules/recursion : Examples of Fun with Recursive functions in Nu (#717)

This directory contains some examples of running recursive algorithms in
Nu. It also has the module : tramp which implements the Trampoline
pattern to overcome the lack of Tail Call Optimization (TCO) avoiding
unlimited stack growth.

---------

Co-authored-by: Darren Schroeder <343840+fdncred@users.noreply.github.com>

modules/recursion/merge.nu

@@ -0,0 +1,36 @@
+# merge 2 sorted lists
+
+# Merge 2 sorted lists
+def merge-2 [l: list, r: list] -> list {
+  mut ol = []
+mut lprime = $l; mut rprime = $r
+  let mx = ($l | length) + ($r | length)
+  #print -e $"l: ($l), r: ($r)"
+  while ($ol | length) < $mx {
+  if ($lprime | is-empty) or ($rprime | is-empty) { break }
+    if $lprime.0 <= $rprime.0 {
+
+      $ol = ($ol | append $lprime.0)
+      $lprime = ($lprime | skip)
+    } else {
+      $ol = ($ol | append $rprime.0)
+    $rprime = ($rprime | skip)
+    }
+  }
+  $ol | append $lprime | append $rprime
+}
+
+
+# Merge sort a list
+# This version is non tail call optimized and might blow the stack for
+# large lists.
+def sort-nontail [x: list] -> list {
+  let $n = ($x | length)
+ let n_2: int = $n // 2
+
+  if $n <= 1 {
+    $x
+  } else {
+    merge-2 (sort-nontail ($x | first $n_2)) (sort-nontail ($x | skip $n_2))
+  }
+}