rbuf.go
func (b *FixedSizeRingBuf) ContigLen() int {
	extent := b.Beg + b.Readable
	firstContigLen := intMin(extent, b.N) - b.Beg
	return firstContigLen
}
func (b *FixedSizeRingBuf) ContigLen() int {
	extent := b.Beg + b.Readable
	firstContigLen := intMin(extent, b.N) - b.Beg
	return firstContigLen
}
// ContigLen gets the length of the largest read that we can provide to a contiguous slice
// without an extra linearizing copy of all bytes internally.