[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/shader_recompiler/frontend/maxwell/structured_control_flow.cpp

@@ -7,7 +7,7 @@
 #include <algorithm>
 #include <memory>
 #include <string>
-#include <ankerl/unordered_dense.h>
+#include "common/container/unordered_map.h"
 #include <utility>
 #include <vector>
 
@@ -390,7 +390,7 @@ private:
                    std::optional<Node> return_label) {
         Statement* const false_stmt{pool.Create(Identity{}, IR::Condition{false}, &root_stmt)};
         Tree& root{root_stmt.children};
-        ankerl::unordered_dense::map<Flow::Block*, Node> local_labels;
+        ::Common::unordered_map<Flow::Block*, Node> local_labels;
         local_labels.reserve(function.blocks.size());
 
         for (Flow::Block& block : function.blocks) {