[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/core/hle/kernel/k_process.h

@@ -78,7 +78,7 @@ private:
     std::array<DebugWatchpoint, Core::Hardware::NUM_WATCHPOINTS> m_watchpoints{};
     std::map<KProcessAddress, u64> m_debug_page_refcounts{};
 #ifdef HAS_NCE
-    ankerl::unordered_dense::map<u64, u64> m_post_handlers{};
+    ::Common::unordered_map<u64, u64> m_post_handlers{};
 #endif
     std::unique_ptr<Core::ExclusiveMonitor> m_exclusive_monitor;
     Core::Memory::Memory m_memory;
@@ -494,7 +494,7 @@ public:
     static void Switch(KernelCore& kernel, KProcess* cur_process, KProcess* next_process);
 
 #ifdef HAS_NCE
-    ankerl::unordered_dense::map<u64, u64>& GetPostHandlers() noexcept {
+    ::Common::unordered_map<u64, u64>& GetPostHandlers() noexcept {
         return m_post_handlers;
     }
 #endif