High number of transitive includes causing bad compile times

I really like the implementation quality of this library. However, consider the following test program:
```cpp
#ifdef STD_UNORDERED
#include
#endif
#ifdef BST_UNORDERED_FWD
#include
#endif
#ifdef BST_UNORDERED
#include
#endif

int main()
{
}
```
and observe the compile time with different preprocessor configurations (after prewarming the filesystem cache):
```
λ time g++-12 -std=c++20 -O2 -I/home/me/ldevel/vcpkg/installed/x64-linux/include compile-test.cpp -o compile-test
g++-12 -std=c++20 […] 0.01s user 0.02s system 99% cpu 0.033 total
λ time g++-12 -std=c++20 -I/home/me/ldevel/vcpkg/installed/x64-linux/include -O2 -DSTD_UNORDERED compile-test.cpp -o compile-test
g++-12 -std=c++20 […] 0.15s user 0.02s system 99% cpu 0.170 total
λ time g++-12 -std=c++20 -O2 -DBST_UNORDERED_FWD -I/home/me/ldevel/vcpkg/installed/x64-linux/include compile-test.cpp -o compile-test
g++-12 -std=c++20 […] 0.49s user 0.02s system 99% cpu 0.517 total
λ time g++-12 -std=c++20 -O2 -DBST_UNORDERED -I/home/me/ldevel/vcpkg/installed/x64-linux/include compile-test.cpp -o compile-test
g++-12 -std=c++20 […] 0.75s user 0.03s system 99% cpu 0.782 total
```
| flags | time |
| ------------- | ------------- |
| | `0.033s` |
| `-DSTD_UNORDERED` | `0.170s` |
| `-DBST_UNORDERED_FWD` | `0.517s` |
| `-DBST_UNORDERED` | `0.782s` |

System information:
* g++-12 (Ubuntu 12.1.0-2ubuntu1~22.04) 12.1.0
* CPU: Intel(R) Core(TM) i9-9980HK

The issue seems to be that `unordered_flat_map` always includes `boost/container_hash/hash.hpp` which in turn includes large parts of the standard library (``, ` => `, `variant`).

It seems somewhat unfortunate to me that the compile time is increased that much, especially since the `unordered_flat_map` is usually part of a composite data structure and therefore transitively included in many TUs.

Furthermore I usually don't use `boost::hash` but a custom hash implementation (SpookyHashv2, xxHash or domain specific hashes) and therefore would like to ask whether it'd be possible to provide a variant which either uses `std::hash` as default `Hash` parameter or doesn't supply one at all.