Avoid warnings from bare trait objects

Since we support Rust 1.18 but modern Rust versions have a warning
about bare trait objects, we go ahead and allow them in the code of
`src/lib.rs` & `snafu-derive/src/lib.rs`. We also have to allow
unknown lints because Rust 1.18 doesn't know about the lint we are
allowing.

We can use `dyn` in the documentation and tests because those are
only executed in modern Rust versions.

snafu-derive/src/lib.rs

@@ -1,3 +1,4 @@
+#![allow(unknown_lints, bare_trait_objects)]
 #![recursion_limit = "128"] // https://github.com/rust-lang/rust/issues/62059
 
 extern crate proc_macro;

src/lib.rs

@@ -1,3 +1,4 @@
+#![allow(unknown_lints, bare_trait_objects)]
 #![deny(missing_docs)]
 
 //! # SNAFU

tests/backtrace.rs

@@ -2,7 +2,7 @@ extern crate snafu;
 
 use snafu::{ensure, Backtrace, ErrorCompat, ResultExt, Snafu};
 
-type AnotherError = Box<std::error::Error>;
+type AnotherError = Box<dyn std::error::Error>;
 
 #[derive(Debug, Snafu)]
 enum Error {

tests/generics.rs

@@ -2,7 +2,7 @@ extern crate snafu;
 
 use snafu::{Backtrace, ResultExt, Snafu};
 
-type BoxError = Box<std::error::Error>;
+type BoxError = Box<dyn std::error::Error>;
 
 #[derive(Debug, Snafu)]
 enum Error<'a, 'x, A, Y> {

tests/source_attributes.rs

@@ -67,7 +67,7 @@ mod transformation {
 
         TransformationToTraitObject {
             #[snafu(source(from(InnerError, Box::new)))]
-            source: Box<std::error::Error>,
+            source: Box<dyn std::error::Error>,
         },
     }