Move changes from exp2 to main

This is largely a move to simplify a lot of the readers
Also further breaks out functions.

internal/metadata/reader.go

@@ -0,0 +1,60 @@
+package metadata
+
+import (
+	"encoding/binary"
+	"io"
+
+	"github.com/CalebQ42/squashfs/internal/decompress"
+)
+
+type Reader struct {
+	master io.Reader
+	cur    io.Reader
+	d      decompress.Decompressor
+}
+
+func NewReader(r io.Reader, d decompress.Decompressor) (*Reader, error) {
+	var out Reader
+	out.d = d
+	out.master = r
+	return &out, out.Advance()
+}
+
+func (r *Reader) Advance() error {
+	if clr, ok := r.cur.(io.Closer); ok {
+		clr.Close()
+	}
+	var size uint16
+	err := binary.Read(r.master, binary.LittleEndian, &size)
+	if err != nil {
+		return err
+	}
+	comp := size&0x8000 != 0x8000
+	size &^= 0x8000
+	r.cur = io.LimitReader(r.master, int64(size))
+	if comp {
+		r.cur, err = r.d.Reader(r.cur)
+		if err != nil {
+			return err
+		}
+	}
+	return nil
+}
+
+func (r Reader) Read(p []byte) (n int, err error) {
+	n, err = r.cur.Read(p)
+	if err == io.EOF {
+		err = r.Advance()
+		if err != nil {
+			return
+		}
+		var tmpN int
+		tmp := make([]byte, len(p)-n)
+		tmpN, err = r.Read(tmp)
+		for i := range tmp {
+			p[n+i] = tmp[i]
+		}
+		n += tmpN
+	}
+	return
+}