[#2] Added Licensing.

I decided on the Apache-2.0 license and have stamped all the files to
now point to that license.

tests/constants.rs

@@ -1,3 +1,6 @@
+// SPDX-License-Identifier: Apache-2.0
+// Sealed with Magistamp.
+
 use std::{f32, f64};
 use sigils::Constants;
 

tests/lib.rs

@@ -1,2 +1,5 @@
+// SPDX-License-Identifier: Apache-2.0
+// Sealed with Magistamp.
+
 mod constants;
 mod vector;

tests/vector.rs

@@ -1,3 +1,6 @@
+// SPDX-License-Identifier: Apache-2.0
+// Sealed with Magistamp.
+
 use std::ops::{Add, Sub, Mul, Div, Rem};
 use std::ops::{AddAssign, SubAssign, MulAssign, DivAssign, RemAssign};
 use sigils::vector::*;