Proposal: log binary data for Solidity

The program_id is not needed on "Program return data: " because it
always preceeded by the program invoke message, so no need to repeat
the program id. Also rename this to "Program return: " since "data"
is redundant.

programs/bpf/c/src/log_data/log_data.c

@@ -0,0 +1,28 @@
+/**
+ * @brief Example C-based BPF program uses sol_log_data
+ */
+#include <solana_sdk.h>
+
+static const uint8_t return_data[] = { 0x08, 0x01, 0x44 };
+
+extern uint64_t entrypoint(const uint8_t *input) {
+  SolAccountInfo ka[1];
+  SolParameters params = (SolParameters) { .ka = ka };
+  SolBytes fields[2];
+
+  if (!sol_deserialize(input, &params, SOL_ARRAY_SIZE(ka))) {
+    return ERROR_INVALID_ARGUMENT;
+  }
+
+  // Generate two fields, split at the first 0 in the input
+  fields[0].addr = params.data;
+  fields[0].len = sol_strlen((char*)fields[0].addr);
+  fields[1].addr = fields[0].addr + fields[0].len + 1;
+  fields[1].len = params.data_len - fields[0].len - 1;
+
+  sol_set_return_data(return_data, sizeof(return_data));
+
+  sol_log_data(fields, 2);
+
+  return SUCCESS;
+}