Add NamedTuple.types to return named tuple of types

spec/std/named_tuple_spec.cr

@@ -296,4 +296,9 @@ describe "NamedTuple" do
     tup = {a: 1, b: 'a'}
     tup.values.should eq({1, 'a'})
   end
+
+  it "does types" do
+    tuple = {a: 1, b: 'a', c: "hello"}
+    tuple.class.types.to_s.should eq("{a: Int32, b: Char, c: String}")
+  end
 end

src/named_tuple.cr

@@ -168,6 +168,16 @@ struct NamedTuple
     hasher
   end
 
+  # Returns the types of this named tuple type.
+  #
+  # ```
+  # tuple = {a: 1, b: "hello", c: 'x'}
+  # tuple.class.types # => {a: Int32, b: String, c: Char}
+  # ```
+  def self.types
+    NamedTuple.new(**{{T}})
+  end
+
   # Same as `to_s`.
   def inspect
     to_s