[common] Switch to boost::unordered_flat containers (#4326)

Replaces all instances of ankerl's unordered map/set with boost's
`unordered_flat_*` classes. This uses std::hash since boost::hash is
actually a lot slower.

Also adds an abstraction layer in `Common` so future changes are quicker
and easier.

Other implementation details:
- ankerl provided hash specializations for tuple and pair, so those were
  ported here
- std::erase_if doesn't work on boost, so just used the ADL'd erase_if

This should be about equal or superior performance as unordered_dense for everything except iteration.

Signed-off-by: crueter <crueter@eden-emu.dev>

- [x] I have read and followed the [Contribution Guidelines](https://git.eden-emu.dev/eden-emu/eden/src/branch/master/CONTRIBUTING.md#code-contributions).
- [x] I have read and followed the [AI Policy](https://git.eden-emu.dev/eden-emu/eden/src/branch/master/docs/policies/AI.md)
- [x] I have read and followed the [Coding Guidelines](https://git.eden-emu.dev/eden-emu/eden/src/branch/master/docs/policies/Coding.md) to the best of my ability.

-------------------

Reviewed-on: https://git.eden-emu.dev/eden-emu/eden/pulls/4326
Reviewed-by: Lizzie and Samuel <lizzie@eden-emu.dev>
Reviewed-by: MaranBr <maranbr@eden-emu.dev>

src/common/host_memory.cpp

@@ -7,7 +7,7 @@
 #ifdef _WIN32
 
 #include <iterator>
-#include <ankerl/unordered_dense.h>
+#include "common/container/unordered_map.h"
 #include <boost/icl/separate_interval_set.hpp>
 #include <windows.h>
 #include "common/dynamic_library.h"
@@ -391,7 +391,7 @@ private:
 
     std::mutex placeholder_mutex;                                 ///< Mutex for placeholders
     boost::icl::separate_interval_set<size_t> placeholders;       ///< Mapped placeholders
-    ankerl::unordered_dense::map<size_t, size_t> placeholder_host_pointers; ///< Placeholder backing offset
+    ::Common::unordered_map<size_t, size_t> placeholder_host_pointers; ///< Placeholder backing offset
 };
 
 #elif defined(__OPENORBIS__) || defined(__managarm__)