Conventional install locations

[AntoinePrv]

• CMake config files are typically installed under ${CMAKE_INSTALL_LIBDIR}/cmake/${PROJECT_NAME}, not the other way around;
• LIBRARY DESTINATION and ARCHIVE_DESTINATION should default to CMAKE_INSTALL_LIBDIR (not CMAKE_INSTALL_LIBDIR/nanobench), which is the default when omited.
• Add NANOBENCH_BUILD_TEST to control building of tests

[AntoinePrv]

@martinus seeing build errors like the following, did this PR trigger them?

/tmp/cirrus-ci-build/src/test/unit_to_s.cpp:27:18: error: 'UINT64_C' was not declared in this scope
   27 |     REQUIRE(to_s(UINT64_C(123456789)) == "123456789");