Cleanup warnings with GCC/Clang (-Wconversion, -Wextra)

[GaspardKirira]

Hi,

I would like to contribute a small warning cleanup for Asio.

Environment:

• Compiler: GCC 13 / Clang 16
• Platform: Linux
• C++ standard: C++20
• Flags: -Wall -Wextra -Wconversion -Wshadow

I found several warnings in:

• include/asio/...

My goal is to submit small, focused PRs that fix one category of warnings at a time, without changing behavior.

Would this kind of contribution be accepted ?

Thanks.