Add benchmark to write JSON into a string

The benchmark BlockToJsonVerbose only tests generating (and destroying)
the JSON data structure, but serializing into a string is also a
performance critical aspect of the RPC calls.

Also, use ankerl::nanobench::doNotOptimizeAway to make sure the compiler
can't optimize the result of the calls away.

src/bench/rpc_blockchain.cpp

@@ -12,25 +12,49 @@
 
 #include <univalue.h>
 
-static void BlockToJsonVerbose(benchmark::Bench& bench)
+namespace {
-{
-    TestingSetup test_setup{};
 
+struct TestBlockAndIndex {
+    TestingSetup test_setup{};
+    CBlock block{};
+    uint256 blockHash{};
+    CBlockIndex blockindex{};
+
+    TestBlockAndIndex()
+    {
         CDataStream stream(benchmark::data::block413567, SER_NETWORK, PROTOCOL_VERSION);
         char a = '\0';
         stream.write(&a, 1); // Prevent compaction
 
-    CBlock block;
         stream >> block;
 
-    CBlockIndex blockindex;
+        blockHash = block.GetHash();
-    const uint256 blockHash = block.GetHash();
         blockindex.phashBlock = &blockHash;
         blockindex.nBits = 403014710;
+    }
+};
 
+} // namespace
+
+static void BlockToJsonVerbose(benchmark::Bench& bench)
+{
+    TestBlockAndIndex data;
     bench.run([&] {
-        (void)blockToJSON(block, &blockindex, &blockindex, /*verbose*/ true);
+        auto univalue = blockToJSON(data.block, &data.blockindex, &data.blockindex, /*verbose*/ true);
+        ankerl::nanobench::doNotOptimizeAway(univalue);
     });
 }
 
 BENCHMARK(BlockToJsonVerbose);
+
+static void BlockToJsonVerboseWrite(benchmark::Bench& bench)
+{
+    TestBlockAndIndex data;
+    auto univalue = blockToJSON(data.block, &data.blockindex, &data.blockindex, /*verbose*/ true);
+    bench.run([&] {
+        auto str = univalue.write();
+        ankerl::nanobench::doNotOptimizeAway(str);
+    });
+}
+
+BENCHMARK(BlockToJsonVerboseWrite);