bench: Add benchmark to write JSON into a string

[martinus]

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.

Extracts test setup into a struct TestBlockAndIndex, and uses it in
both BlockToJsonVerbose and BlockToJsonVerboseWrite.

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

Here are benchmark results on my Intel i7-8700:

ns/op	op/s	err%	ins/op	cyc/op	IPC	bra/op	miss%	total	benchmark
71,807,017.00	13.93	0.4%	555,782,961.00	220,788,645.00	2.517	102,279,341.00	0.4%	0.80	BlockToJsonVerbose
27,916,835.00	35.82	0.1%	235,084,034.00	89,033,525.00	2.640	42,911,139.00	0.3%	0.32	BlockToJsonVerboseWrite

[DrahtBot]

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Conflicts

Reviewers, this pull request conflicts with the following ones:

• rpc: Add level 3 verbosity to getblock RPC call. #21245 (rpc: Add level 3 verbosity to getblock RPC call. by fyquah)
• test: Move MakeNoLogFileContext to libtest_util, and use it in bench #21003 (test: Move MakeNoLogFileContext to libtest_util, and use it in bench by MarcoFalke)

If you consider this pull request important, please also help to review the conflicting pull requests. Ideally, start with the one that should be merged first.

[practicalswift]

Concept ACK

Really like nanobench BTW: a really nice addition to Bitcoin Core!

[martinus]

Really like nanobench BTW: a really nice addition to Bitcoin Core!

Thanks! Probably not widely known, but it is really helpful to run a benchmark in an endless loop like so:

NANOBENCH_ENDLESS=BlockToJsonVerbose ./src/bench/bench_bitcoin -filter=BlockToJsonVerbose

And then fire up a profiler like perf top or hotspot to see where all the cycles are used up :)

[laanwj]

Code review ACK e3e0a24