This prevents the generation of these headers from also depending on the
dependencies of the libs/binaries which consume them.
Specifically, this prevents generated test headers (such as
test/data/base58_encode_decode.json.h) from depending on the
dependencies of test_bitcoin (libcrc32c.a libcrc32c_sse42.a libleveldb.a)
Note that this is currently only relevant for Ninja.
For more detail, see:
https://cmake.org/cmake/help/latest/command/add_custom_command.html
This change allows to drop brittle sizeof calls in favor of the
std::span::size method.
Other improvements include:
* Use of a namespace to mark test and bench data
* Use of the modern std::byte
* Drop of a no longer used std::vector copy and the bench/data module