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/Cargo.toml

@@ -42,6 +42,7 @@ assert_matches = { version = "1.5.0", optional = true }
 bincode = "1.3.3"
 bytemuck = { version = "1.7.2", features = ["derive"] }
 borsh = "0.9.0"
+base64 = "0.13"
 borsh-derive = "0.9.0"
 bs58 = "0.4.0"
 bv = { version = "0.11.1", features = ["serde"] }