In 32-bit builds, the number of hexadecimal digits for the chunk header can be reduced to 8

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There is no practical way we could have ended up writing a chunk with a size larger than 2^32-1 on 32-bit machines because we cannot have a buffer with such a size. This presents an optimization opportunity to reduce the number of hexadecimal digits to 8 for writing the chunk header: https://github.com/cppalliance/http_proto/blob/39a71bf6c0bb042f490239d339ca81caf8f370b8/src/serializer.cpp#L56