[codethinki]

Depends on codethinki/cth_cmake#8. lib/cth_cmake points at the commit of that PR branch; after #8 is merged it has to move to the merged commit before this PR is merged.

• gtl becomes a dependency of the cth component: gtl in vcpkg.json, and cth/cmake/Findgtl.cmake (gtl installs only its headers and no CMake package), registered with cth_pkg_add_find_modules and found with cth_pkg_target_find_package(cth gtl MODULE REQUIRED). The installed package ships the module, so consumers of the installed cth need nothing but gtl.
• new cth/data/hash_map.hpp: cth::dt::flat_hash_map and cth::dt::flat_hash_set, aliases of the gtl types (no pointer stability)
• std unordered containers replaced:
  • data/pool.hpp, data/thread_pool.hpp, the thread pool test and cth_net websocket server: the flat gtl types
  • data/union_find.hpp all(): a std::vector indexed by the root, the roots are indices and all() stays constexpr
  • algorithm/combine.hpp: the node map becomes a flat_hash_map; the element pool stays std::unordered_set, it keeps pointers to its elements and needs the pointer stability

Tested: gcc_debug build without errors or warnings, all 272 tests pass. The installed package works from a consumer with find_package(cth CONFIG REQUIRED COMPONENTS cth) and the real gtl. cth_engine builds against this branch (with cth_cmake#8) and its 318 tests pass.