IWYU - Clean up unused includes and forward declarations

[KaganCanSit]

Hello,

I am aware that improving the project's compilation time and cleanliness is currently a high priority. With the help of IWYU, I have identified some points. I hope this helps.

Frankly, I'm not sure about some points. I'll mark them as draft. (wmmintrin/immintrin)

Changes:

• Replace <wmmintrin.h> with <immintrin.h> in aes_vaes.cpp (covers VAES 256-bit intrinsics)
• Replace <immintrin.h> with <emmintrin.h> in zfec_sse2.cpp (SSE2-only module)
• Remove redundant <cstdint> includes where botan/types.h provides coverage
• Remove unused <memory>, <string>, <algorithm>, <cstddef> includes
• Remove dead forward declarations: Barrett_Reduction, Kyber_Keypair_Codec, Sphincs_Address, BLAKE2bMAC, Classic_McEliece_Polynomial_Ring, Classic_McEliece_Big_F_Coefficient, EC_Group_Data, RandomNumberGenerator

I didn't encounter any errors during compilation and testing in the Linux environment, but I will look at the CI. Additionally, if there is any point you would like me to revise, please leave a comment and I will look into it.

Regards.

---

Note: Additionally, I saw the following warnings. It compiles when I make changes, but I didn't want to add them because I wasn't sure.

//build/include/public/botan/p11_randomgenerator.h should remove these lines:
- namespace Botan { namespace PKCS11 { class Module; } }
//build/include/public/botan/p11_object.h should remove these lines:
- namespace Botan { namespace PKCS11 { class Module; } }
 
// Already defines inside the p11_types.h

[coveralls]

coverage: 90.335% (-0.001%) from 90.336%
when pulling cc6ba53 on KaganCanSit:iwyu-header-and-forward-declaration-cleanup
into 90b85a8 on randombit:master.

[KaganCanSit]

Hit by #5382, rebase to master branch.