Splice ./universe directory into ./

Manually merging:
- README.md: I added the description from universe/README.md into the heading of
  dotfiles/README.md.
- .envrc: dotfiles/.envrc was a superset of universe/.envrc
- .gitignore: Adding some of the ignored patterns from universe/.gitignore to
  dotfiles/.gitignore

Everything else here should be a simple rename.

universe/data_structures_and_algorithms/find-unique-int-among-duplicates.py

@@ -1,45 +0,0 @@
-import unittest
-
-
-################################################################################
-# Solution
-################################################################################
-def find_unique_delivery_id(xs):
-    a = 0
-    for x in xs:
-        a ^= x
-    return a
-
-
-################################################################################
-# Tests
-################################################################################
-class Test(unittest.TestCase):
-    def test_one_drone(self):
-        actual = find_unique_delivery_id([1])
-        expected = 1
-        self.assertEqual(actual, expected)
-
-    def test_unique_id_comes_first(self):
-        actual = find_unique_delivery_id([1, 2, 2])
-        expected = 1
-        self.assertEqual(actual, expected)
-
-    def test_unique_id_comes_last(self):
-        actual = find_unique_delivery_id([3, 3, 2, 2, 1])
-        expected = 1
-        self.assertEqual(actual, expected)
-
-    def test_unique_id_in_middle(self):
-        actual = find_unique_delivery_id([3, 2, 1, 2, 3])
-        expected = 1
-        self.assertEqual(actual, expected)
-
-    def test_many_drones(self):
-        actual = find_unique_delivery_id(
-            [2, 5, 4, 8, 6, 3, 1, 4, 2, 3, 6, 5, 1])
-        expected = 8
-        self.assertEqual(actual, expected)
-
-
-unittest.main(verbosity=2)