func (sb *SectorBuilder) SliceForAppend() []byte {
	return sb.sector[sb.sectorLen:]
}
SliceForAppend returns a slice into the unused capacity of the sector. This
makes it possible to Append to the sector without allocating new memory.
package renter
import (
	"errors"
	"fmt"
	"go.sia.tech/siad/crypto"
	"go.sia.tech/siad/types"
	"lukechampine.com/frand"
	"lukechampine.com/us/hostdb"
	"lukechampine.com/us/merkle"
	"lukechampine.com/us/renter/proto"
	"lukechampine.com/us/renterhost"
)
type SectorBuilder struct {
	sector    [renterhost.SectorSize]byte
	sectorLen int
	slices    []SectorSlice
}
func (sb *SectorBuilder) Reset() {
	sb.sectorLen = 0
	sb.slices = nil  
}