Optimizing mul function in diyp

[wjr-z]

I noticed that the mul function in diyfp still uses a simple algorithm. In fact, simple inline assembly can be used in clang and GCC, and in MSVC__ UMULH for optimization.

[nlohmann]

Looking into it.

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This reply was drafted by Claude Code on behalf of @nlohmann, as part of a review of open discussions.