func (c Key) Put(x uint64) {
	s := uint(8 * (len(c) - 1))
	for i := range c {
		c[i] = byte(x >> s)
		s -= 8
	}
}
Put serializes x into the buffer (big endian). Behavior is undefined when x
does not fit, so the caller must ensure c is large enough.
package nuts
func KeyLen(x uint64) int {
	n := 1
	if x >= 1<<32 {
		x >>= 32
		n += 4
	}
	if x >= 1<<16 {
		x >>= 16
		n += 2
	}
	if x >= 1<<8 {
		x >>= 8
		n += 1
	}
	return n
}
type Key []byte