In my micro-benchmarks memcmp(4196) 3 times faster than old
implementation. Also, it's generally better to use as less
reinterpret_casts<> as possible.

buf_is_zeroes(): renamed from buf_page_is_zeroes() and
argument changed to span<> for convenience.

st_::span<T>::const_iterator: fixed

page_zip-verify_checksum(): make argument byte* instead of void*