Let´s see if someone is still here to merge it. Overall maintenance patch with updated dependencies and C++20 compliance. Only fixes compiler errors, no warnings for now. Changes:

• updates googletest (closes Incompatibility with compiler due to outdated Library  #211)
• don´t build googletest if THRILL_BUILD_TESTS is off, prevents clash with clients using their own googletest version
• updates cereal (closes the version of cereal is very old, need to update  #204)
• updates tlx for C++20 (enabled by Increase compatibility with C++20 tlx/tlx#30)
• moves SeqReordering to header as compiler complains about incomplete type

Builds (including THRILL_BUILD_TESTS and THRILL_BUILD_EXAMPLES) work for C++14 up to C++20, tested with gcc 12.3.0 and clang 15.0.7.