Forbid unsafe_code

[fintelia]

Fixes #222. Contrary to what is mentioned there, removing this annotation would not constitute a breaking change requiring a major version bump (at least according to normal Rust conventions). It is merely an assertion that the current patch release contains no unsafe code.

Looking for #![forbid(unsafe_code)] is more reliable than trying to grep the crate source code for instances of "unsafe", since the latter could hypothetically be concealed by macros and such.

[KodrAus]

Thanks @fintelia 👍 I think this is something I'd be comfortable doing now. My comment about major version bumps was less about semver compatibility and more about making a reasonable commitment to users who care about the presence of unsafe code in their dependencies. You certainly should expect unsafe code to appear or disappear in patches, but I see #[forbid(unsafe_code)] also as a design guideline to avoid any solutions that require unsafe code.