[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/android/app/src/main/jni/game_metadata.cpp

@@ -20,7 +20,7 @@ struct RomMetadata {
     std::vector<u8> icon;
     bool isHomebrew;
 };
-static ankerl::unordered_dense::map<std::string, RomMetadata> m_rom_metadata_cache;
+static ::Common::unordered_map<std::string, RomMetadata> m_rom_metadata_cache;
 
 static RomMetadata CacheRomMetadata(const std::string& path) {
     auto& instance = EmulationSession::GetInstance();

src/android/app/src/main/jni/native_input.cpp

@@ -21,7 +21,7 @@
 #include "input_common/drivers/virtual_gamepad.h"
 #include "native.h"
 
-ankerl::unordered_dense::map<std::string, std::unique_ptr<AndroidConfig>> map_profiles;
+::Common::unordered_map<std::string, std::unique_ptr<AndroidConfig>> map_profiles;
 
 bool IsHandheldOnly() {
     const auto npad_style_set =