Add Turning Things Into Hashes as a ruby til

ruby/turning-things-into-hashes.md

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+# Turning Things Into Hashes
+
+We have `#to_h` for turning things into hashes.
+
+It works as an identity function:
+
+```ruby
+> {}.to_h
+=> {}
+> {hello: "world"}.to_h
+=> {:hello=>"world"}
+```
+
+It works with `nil`:
+
+```ruby
+> nil.to_h
+=> {}
+```
+
+Does it work with arrays?
+
+```ruby
+> [:one, 2].to_h
+TypeError: wrong element type Symbol at 0 (expected array)
+from (pry):36:in `to_h'
+```
+
+Yes, but only if it is an array of pairs:
+
+```ruby
+> [[:one, 2], [:three, 4]].to_h
+=> {:one=>2, :three=>4}
+```
+
+It also works with `Struct` and `OpenStruct`:
+
+```
+> Person = Struct.new(:name, :age)
+=> Person
+> bob = Person.new("bob", 45)
+=> #<struct Person name="bob", age=45>
+> bob.to_h
+=> {:name=>"bob", :age=>45}
+```
+
+You'll find that many other objects and gems support `#to_h` when it makes
+sense.