io: Add SPDX identifier

The `io` crate is licensed in the manifest using the CC0-1.0 license
same as the rest of the codebase but none of the individual files have a
license blurb.

Add a CC0-1.0 license blurb by way of an SPDX-License-Identifier tag.

io/src/bridge.rs

@@ -1,3 +1,5 @@
+// SPDX-License-Identifier: CC0-1.0
+
 #[cfg(feature = "alloc")]
 use alloc::boxed::Box;
 

io/src/error.rs

@@ -1,3 +1,5 @@
+// SPDX-License-Identifier: CC0-1.0
+
 #[cfg(all(not(feature = "std"), feature = "alloc"))]
 use alloc::boxed::Box;
 use core::fmt;

io/src/lib.rs

@@ -1,3 +1,5 @@
+// SPDX-License-Identifier: CC0-1.0
+
 //! Rust-Bitcoin I/O Library
 //!
 //! The `std::io` module is not exposed in `no-std` Rust so building `no-std` applications which

io/src/macros.rs

@@ -1,3 +1,5 @@
+// SPDX-License-Identifier: CC0-1.0
+
 #[macro_export]
 /// Because we cannot provide a blanket implementation of [`std::io::Write`] for all implementers
 /// of this crate's `io::Write` trait, we provide this macro instead.