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/rust/invoked/src/processor.rs

@@ -8,7 +8,7 @@ use solana_program::{
     bpf_loader, entrypoint,
     entrypoint::{ProgramResult, MAX_PERMITTED_DATA_INCREASE},
     msg,
-    program::{invoke, invoke_signed},
+    program::{get_return_data, invoke, invoke_signed, set_return_data},
     program_error::ProgramError,
     pubkey::Pubkey,
     system_instruction,
@@ -27,6 +27,8 @@ fn process_instruction(
         return Ok(());
     }
 
+    assert_eq!(get_return_data(), None);
+
     match instruction_data[0] {
         VERIFY_TRANSLATIONS => {
             msg!("verify data translations");
@@ -286,6 +288,11 @@ fn process_instruction(
                 }
             }
         }
+        SET_RETURN_DATA => {
+            msg!("Set return data");
+
+            set_return_data(b"Set by invoked");
+        }
         _ => panic!(),
     }