Fixes error C4426 (optimization flags changed after including header) on MSVC

[gamecentric]

According to Microsoft Learn the effects of pragma optimize with an empty string "" are the following

• When you use the off parameter, it turns all the optimizations, g, s, t, and y, off.
• When you use the on parameter, it resets the optimizations to the ones that you specified using the /O compiler option.

With the original code, the result of STDEXEC_PRAGMA_OPTIMIZE_BEGIN is always a no-op, while the effects of STDEXEC_PRAGMA_OPTIMIZE_END is to disable all optimizations. This makes compilation fail with a C4426 error if the library is included in a project where optimizations are enabled.
In MSVC the pragma cannot set a perfect equivalent of GCC "/O3" optimization level, so "t" (favor fast code) has been chosen as an alternative.

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[ericniebler]

/ok to test 921e01f

[ericniebler]

thanks!