putter.go
func growPartSize(partIndex int, partSize, putsz int64) bool {
	return (maxObjSize-putsz)/(maxNPart-int64(partIndex)) > partSize
}
func growPartSize(partIndex int, partSize, putsz int64) bool {
	return (maxObjSize-putsz)/(maxNPart-int64(partIndex)) > partSize
}
// returns true unless partSize is large enough
// to achieve maxObjSize with remaining parts