feat(Complex): add an I constant, to allow I * 4. + 3. constructors

Note that `3. + 4. * I` doesn't work. See rust-lang/rust#19035

Author: Jorge Aparicio

src/f32.rs

@@ -0,0 +1,5 @@
+//! Single precision
+
+use Complex;
+
+pub const I: Complex<f32> = Complex { re: 0., im: 1. };

src/f64.rs

@@ -0,0 +1,5 @@
+//! Double precision
+
+use Complex;
+
+pub const I: Complex<f64> = Complex { re: 0., im: 1. };

src/lib.rs

@@ -1,6 +1,7 @@
 //! Complex numbers
 
 #![deny(missing_docs, warnings)]
+#![feature(macro_rules)]
 
 extern crate onezero;
 
@@ -10,6 +11,9 @@ use std::rand::{Rand, Rng};
 
 use onezero::{One, Zero};
 
+pub mod f32;
+pub mod f64;
+
 /// A complex number in Cartesian form.
 #[repr(C)]
 #[deriving(PartialEq)]
@@ -194,3 +198,22 @@ impl<T> Zero for Complex<T> where T: Zero {
         }
     }
 }
+
+#[cfg(test)]
+mod test {
+    macro_rules! test {
+        ($($ty:ident),+) => {$(
+            mod $ty {
+                use Complex;
+                use $ty::I;
+
+                #[test]
+                fn constructor() {
+                    assert_eq!(Complex::new(3., 4.), I * 4. + 3.)
+                }
+            })+
+         }
+    }
+
+    test!(f32, f64)
+}