We are currently using old version 0.6.2 of `xxhash`, as a verbatim copy and paste of its header file `xxhash.h`. Switch to the more recent version 0.8.2. Since this version is in Conan Center (and properly protects its ABI by keeping the state object incomplete), add it as a Conan requirement. Switch to the SIMD instructions (in the new `XXH3` family) supported by the new version.