Fix warnings from use of std::accumulate in xbuilder

[bredej]

Checklist

• The title and commit message(s) are descriptive.
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Description

The internal type of std::accumulate is determined by the init parameter.
Initializing with 0 (int) results in a "conversion from 'size_t' to 'type', possible loss of data" warning in MSVC.
Related #2732

The second commit replacing size_t with std::size_t is just for consistency.

[codspeed-hq]

Merging this PR will not alter performance

✅ 255 untouched benchmarks

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_{Comparing bredej:master (bfbf021) with master (dd857e5)}

[JohanMabille]

Thanks!