Add #![forbid(unsafe_code)].

The newly added `#![forbid(unsafe_code)]` directive serves two purposes:

* It explicitly documents the current state of the crate (i.e. currently
  there is no `unsafe`) which helps security reviewers to check the
  overall safety profile / approach of the crate.
* It adds an extra speed bump in case `unsafe` PRs are considered in the
  future. Hopefully in most cases `unsafe` can be encapsulated in
  separate (small, auditable) crates that `qr_code` can depend on (and
  the `forbid` directive can give a gentle nudge in this direction).

src/lib.rs

@@ -15,6 +15,9 @@
 #![cfg_attr(feature = "bench", doc(include = "../README.md"))]
 // ^ make sure we can test our README.md.
 #![cfg_attr(docsrs, feature(doc_cfg))]
+// No `unsafe` please.  If `unsafe` is really needed, then please
+// consider encapsulating it in a separate crates.io crate.
+#![forbid(unsafe_code)]
 
 // Re-exported dependencies.
 #[cfg(feature = "bmp")]