Get rid of unsafe code in the interpreter (#303)

* Get rid of unsafe code unless really necessary

* Add lint levels where appropriate

Some crates (air-beautifier, air-testing-framework) have lot of
rust_2018_idioms violations, that will be resolved later.

crates/air-lib/interpreter-data/src/lib.rs

@@ -14,6 +14,7 @@
  * limitations under the License.
  */
 
+#![forbid(unsafe_code)]
 #![warn(rust_2018_idioms)]
 #![deny(
     dead_code,