Resolve dead_code and unused_variables warnings

Summary:
Rust 1.79's dead code scanner is more precise than previous versions. `pub` is no longer sufficient to hide a data structure field from the lint. It actually looks at whether an unused pub field is reachable from outside the crate.

In the following example, the field `field` is considered dead code by Rust 1.79 and not by 1.78.

```lang=rust
mod module {
    pub struct Struct {
        pub field: i32,
    }

    impl Struct {
        pub fn new() -> Self {
            Struct { field: 0 }
        }
    }
}

pub fn repro() {
    let _ = Struct::new();
}
```

Reviewed By: zertosh, JakobDegen

Differential Revision: D59623034

fbshipit-source-id: 6a4e2fb6e5be410d5127b451704df74ecdd42bf0

reverie-ptrace/src/gdbstub/session.rs

@@ -60,6 +60,7 @@ pub struct Session {
     pub stream_tx: BoxWriter,
 
     /// buffer use to send data over to tcp stream
+    #[allow(dead_code)]
     pub tx_buf: BytesMut,
 
     /// Gdb remote protocol command notifier.