Insert the specified move-insertable buffer before the beginning of this blob. The length of this blob is incremented by the length of the prepended buffer. The buffer is left in a valid but unspecified state. The behavior is undefined unless neither the total size of the resulting blob nor its total number of buffers exceeds INT_MAX. Note that this operation is equivalent to: ` const int n = blob.length(); blob.insertBuffer(0, MoveUtil::move(buffer)); blob.setLength(n + buffer.size()); ` but is more efficient.