Implements #flatten! method (#238)

* Implements #flatten! method

Closes #174

* rubocop

lib/literal/array.rb

@@ -247,6 +247,10 @@ def first(...)
 		@__value__.first(...)
 	end
 
+	def flatten!(...)
+		@__value__.flatten!(...) ? self : nil
+	end
+
 	def freeze
 		@__value__.freeze
 		super

test/array.test.rb

@@ -473,6 +473,18 @@
 	}.to_raise(ArgumentError)
 end
 
+test "#flatten! flattens the array" do
+	array = Literal::Array(Array).new([1, 2], [3, 4])
+
+	expect((array.flatten!).to_a) == [1, 2, 3, 4]
+end
+
+test "#flatten! returns nil if no nested arrays" do
+	array = Literal::Array(Integer).new(1, 2, 3)
+
+	expect(array.flatten!) == nil
+end
+
 test "#flatten flattens the array" do
 	array = Literal::Array(Array).new([1, 2], [3, 4])