Forbid unsafe code (#332)

* Forbid unsafe code

* Add badge advertising that unsafe code is forbidden

* Revert "Add badge advertising that unsafe code is forbidden"

This reverts commit e8d35d20d016a341d37969b975463588d3143528.

* Comment on #![forbid(unsafe_code)]

* Run rustfmt

src/lib.rs

@@ -147,6 +147,11 @@
 #![doc(html_root_url = "https://docs.rs/toml/0.5")]
 #![deny(missing_docs)]
 #![warn(rust_2018_idioms)]
+// Makes rustc abort compilation if there are any unsafe blocks in the crate.
+// Presence of this annotation is picked up by tools such as cargo-geiger
+// and lets them ensure that there is indeed no unsafe code as opposed to
+// something they couldn't detect (e.g. unsafe added via macro expansion, etc).
+#![forbid(unsafe_code)]
 
 pub mod map;
 pub mod value;