[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/dynarmic/tests/A64/fibonacci.cpp

@@ -8,7 +8,7 @@
 
 #include <array>
 #include <exception>
-#include <ankerl/unordered_dense.h>
+#include "common/container/unordered_map.h"
 
 #include <catch2/catch_test_macros.hpp>
 #include "common/common_types.h"
@@ -23,7 +23,7 @@ namespace {
 class MyEnvironment final : public A64::UserCallbacks {
 public:
     u64 ticks_left = 0;
-    ankerl::unordered_dense::map<u64, u8> memory{};
+    ::Common::unordered_map<u64, u8> memory{};
 
     u8 MemoryRead8(u64 vaddr) override {
         return memory[vaddr];