Implements #flatten! method

Closes joeldrapper#174

lib/literal/array.rb

@@ -243,6 +243,10 @@ def first(...)
 		@__value__.first(...)
 	end
 
+	def flatten!(...)
+		@__value__.flatten!(...) ? self : nil
+	end
+
 	def freeze
 		@__value__.freeze
 		super

test/array.test.rb

@@ -472,3 +472,15 @@
 		array.narrow(Numeric)
 	}.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