Move changes from exp2 to main

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

internal/toreader/readerat.go

@@ -0,0 +1,23 @@
+package toreader
+
+import "io"
+
+type ReaderAt struct {
+	d []byte
+}
+
+func NewReaderAt(r io.Reader) (ra ReaderAt, err error) {
+	ra.d, err = io.ReadAll(r)
+	return
+}
+
+func (r ReaderAt) ReadAt(p []byte, off int64) (n int, err error) {
+	if int(off) >= len(r.d) {
+		return 0, io.EOF
+	}
+	n = copy(p, r.d[off:])
+	if n != len(p) {
+		err = io.EOF
+	}
+	return
+}