Fix generated code and update example integration

Resolve `Result` ambiguity and lifetime issues in generated services.
Use `file_stem` for proto filenames. Make `StatusBody` public in
`roto-tonic` and update the `hello_world` build process.

roto-tonic/src/lib.rs

@@ -101,7 +101,7 @@ impl BufferPool {
     }
 }
 
-pub struct StatusBody(pub(crate) Option<Bytes>);
+pub struct StatusBody(pub Option<Bytes>);
 
 impl Body for StatusBody {
     type Data = Bytes;