Add return data implementation

This consists of:
 - syscalls
 - passing return data from invoked to invoker
 - printing to stable log
 - rust and C SDK changes

programs/bpf_loader/src/lib.rs

@@ -917,6 +917,11 @@ impl Executor for BpfExecutor {
                 let trace_string = String::from_utf8(trace_buffer).unwrap();
                 trace!("BPF Program Instruction Trace:\n{}", trace_string);
             }
+            drop(vm);
+            let return_data = invoke_context.get_return_data();
+            if let Some((program_id, return_data)) = return_data {
+                stable_log::program_return_data(&logger, program_id, return_data);
+            }
             match result {
                 Ok(status) => {
                     if status != SUCCESS {