Add return data implementation

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

sdk/bpf/c/inc/sol/return_data.h

@@ -0,0 +1,41 @@
+#pragma once
+/**
+ * @brief Solana return data system calls
+**/
+
+#include <sol/types.h>
+#include <sol/pubkey.h>
+
+#ifdef __cplusplus
+extern "C"
+{
+#endif
+
+/**
+ * Maximum size of return data
+ */
+#define MAX_RETURN_DATA 1024
+
+/**
+ * Set the return data
+ *
+ * @param bytes byte array to set
+ * @param bytes_len length of byte array. This may not exceed MAX_RETURN_DATA.
+ */
+void sol_set_return_data(const uint8_t *bytes, uint64_t bytes_len);
+
+/**
+ * Get the return data
+ *
+ * @param bytes byte buffer
+ * @param bytes_len maximum length of buffer
+ * @param program_id the program_id which set the return data. Only set if there was some return data (the function returns non-zero).
+ * @param result length of return data (may exceed bytes_len if the return data is longer)
+ */
+uint64_t sol_get_return_data(const uint8_t *bytes, uint64_t bytes_len, SolPubkey *program_id);
+
+#ifdef __cplusplus
+}
+#endif
+
+/**@}*/

sdk/bpf/c/inc/solana_sdk.h

@@ -12,6 +12,7 @@
 #include <sol/keccak.h>
 #include <sol/log.h>
 #include <sol/pubkey.h>
+#include <sol/return_data.h>
 #include <sol/secp256k1.h>
 #include <sol/sha.h>
 #include <sol/string.h>