don't bring in the fastcpy_unsafe module when with safe-* features

No functional changes intended.

This makes it build successfully in an environment with `--deny=unsafe_code`, e.g.:
```
RUSTFLAGS=--deny=unsafe_code cargo build
```

src/lib.rs

@@ -88,6 +88,8 @@ pub mod frame;
 
 #[allow(dead_code)]
 mod fastcpy;
+
+#[cfg(not(all(feature = "safe-encode", feature = "safe-decode")))]
 #[allow(dead_code)]
 mod fastcpy_unsafe;