Add several elixir solutions

elixir/grains/.exercism/config.json

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+{
+  "authors": [
+    "rubysolo"
+  ],
+  "contributors": [
+    "andrewsardone",
+    "angelikatyborska",
+    "bunnymatic",
+    "Cohen-Carlisle",
+    "dalexj",
+    "devonestes",
+    "elasticdog",
+    "henrik",
+    "herminiotorres",
+    "jinyeow",
+    "lpil",
+    "neenjaw",
+    "parkerl",
+    "petehuang",
+    "pminten",
+    "sotojuan",
+    "Teapane",
+    "waiting-for-dev"
+  ],
+  "files": {
+    "solution": [
+      "lib/grains.ex"
+    ],
+    "test": [
+      "test/grains_test.exs"
+    ],
+    "example": [
+      ".meta/example.ex"
+    ]
+  },
+  "blurb": "Calculate the number of grains of wheat on a chessboard given that the number on each square doubles.",
+  "source": "JavaRanch Cattle Drive, exercise 6",
+  "source_url": "http://www.javaranch.com/grains.jsp"
+}

elixir/grains/.exercism/metadata.json

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+{"track":"elixir","exercise":"grains","id":"323e0439aa1d482b91d857e239fa16b7","url":"https://exercism.org/tracks/elixir/exercises/grains","handle":"halfdan","is_requester":true,"auto_approve":false}