We were using a second std::vector as a buffer to convert another
std::vector's data into a byte sequence, however we can just use
pointers to the original data and use them directly with WriteBuffer,
which avoids copying the data at all into a separate std::vector.
We simply cast the pointers to u8* (which is allowed by the standard,
given std::uint8_t is an alias for unsigned char on platforms that we
support).
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