Refactor compile_error message

In preparation for enabling rustfmt on `lib.rs` refactor the
`compile_error` call so that the formatter does not touch it.

src/lib.rs

@@ -50,9 +50,10 @@ compile_error!("at least one of the `std` or `no-std` features must be enabled")
 
 // Disable 16-bit support at least for now as we can't guarantee it yet.
 #[cfg(target_pointer_width = "16")]
-compile_error!("rust-bitcoin currently only supports architectures with pointers wider
-                than 16 bits, let us know if you want 16-bit support. Note that we do
-                NOT guarantee that we will implement it!");
+compile_error!(
+    "rust-bitcoin currently only supports architectures with pointers wider than 16 bits, let us
+    know if you want 16-bit support. Note that we do NOT guarantee that we will implement it!"
+);
 
 #[cfg(bench)] extern crate test;