Add zero? to Number, Time::Span, and Complex

spec/std/complex_spec.cr

@@ -168,4 +168,11 @@ describe "Complex" do
     c = Complex.new(4, 6.2)
     c.clone.should eq(c)
   end
+
+  it "test zero?" do
+    Complex.new(0, 0).zero?.should eq true
+    Complex.new(0, 3.4).zero?.should eq false
+    Complex.new(1.2, 0).zero?.should eq false
+    Complex.new(1.2, 3.4).zero?.should eq false
+  end
 end

spec/std/number_spec.cr

@@ -166,6 +166,15 @@ describe "Number" do
     ary[2].should eq(300.to_u8)
   end
 
+  it "test zero?" do
+    0.zero?.should eq true
+    0.0.zero?.should eq true
+    0f32.zero?.should eq true
+    1.zero?.should eq false
+    1.0.zero?.should eq false
+    1f32.zero?.should eq false
+  end
+
   describe "step" do
     it "from int to float" do
       count = 0

spec/std/time/span_spec.cr

@@ -240,4 +240,9 @@ describe Time::Span do
   it "should sum" do
     [1.second, 5.seconds].sum.should eq(6.seconds)
   end
+
+  it "test zero?" do
+    Time::Span.new(0).zero?.should eq true
+    Time::Span.new(123456789).zero?.should eq false
+  end
 end

src/complex.cr

@@ -223,6 +223,10 @@ struct Complex
   def self.zero : Complex
     new 0, 0
   end
+
+  def zero? : Bool
+    @real == 0 && @imag == 0
+  end
 end
 
 struct Number

src/number.cr

@@ -245,6 +245,16 @@ struct Number
     self
   end
 
+  # Returns `true` if value is equal to zero.
+  #
+  # ```
+  # 0.zero? # => true
+  # 5.zero? # => false
+  # ```
+  def zero? : Bool
+    self == 0
+  end
+
   private class StepIterator(T, L, B)
     include Iterator(T)
 

src/time/span.cr

@@ -286,6 +286,10 @@ struct Time::Span
   def self.zero
     new(0)
   end
+
+  def zero?
+    @ticks == 0
+  end
 end
 
 struct Int