Rename exercises dir

Name it instead "topics" so it won't be
strange if some topics included "exercises" directory.

topics/sql/improve_query.md

@@ -0,0 +1,10 @@
+## Comparisons vs. Functions
+
+1. Improve the following query
+
+```
+SELECT count(*)
+FROM shawarma_purchases
+WHERE
+  YEAR(purchased_at) == '2017'
+```

topics/sql/solutions/improve_query.md

@@ -0,0 +1,9 @@
+## Comparisons vs. Functions - Solution
+
+```
+SELECT count(*)
+FROM shawarma_purchases
+WHERE
+  purchased_at >= '2017-01-01' AND
+  purchased_at <= '2017-31-12'
+```