Add Reversing A List as an elixir til

elixir/reversing-a-list.md

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+# Reversing A List
+
+To efficiently work with and _transform_ lists in Elixir, you will likely
+need utilize a list reversing function from time to time. Your best bet is
+to reach for the Erlang implementation which is available as part of the
+`lists` module.
+
+Here are a couple examples of how to use it:
+
+```elixir
+> :lists.reverse([1,2,3])
+[3, 2, 1]
+> :lists.reverse([1, :a, true, "what", 5])
+[5, "what", true, :a, 1]
+```
+
+Note: though I said "_transform_ lists" above, what is actually going on is
+that a new version of the list representing my transformation is being
+created, per Elixir's functional nature.