[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>

.patch/unordered-dense/0001-avoid-memset-when-clearing-an-empty-table.patch

@@ -1,26 +0,0 @@
-From b3622608433c183ba868a1dc8dd9cf285eb3b916 Mon Sep 17 00:00:00 2001
-From: Dario Petrillo <dario.pk1@gmail.com>
-Date: Thu, 27 Nov 2025 23:12:38 +0100
-Subject: [PATCH] avoid extra memset when clearing an empty table
-
----
- include/ankerl/unordered_dense.h | 6 ++++--
- 1 file changed, 4 insertions(+), 2 deletions(-)
-
-diff --git a/include/ankerl/unordered_dense.h b/include/ankerl/unordered_dense.h
-index 0835342..4938212 100644
---- a/include/ankerl/unordered_dense.h
-+++ b/include/ankerl/unordered_dense.h
-@@ -1490,8 +1490,10 @@ class table : public std::conditional_t<is_map_v<T>, base_table_type_map<T>, bas
-     // modifiers //////////////////////////////////////////////////////////////
- 
-     void clear() {
--        m_values.clear();
--        clear_buckets();
-+        if (!empty()) {
-+            m_values.clear();
-+            clear_buckets();
-+        }
-     }
- 
-     auto insert(value_type const& value) -> std::pair<iterator, bool> {