Do a large chunk of the move to go

This commit moves a huge chunk of the code to go. The TODO.md document
indicates what is left to be done.

internal/fsx/copy.go

@@ -0,0 +1,41 @@
+package fsx
+
+import (
+	"errors"
+	"io"
+	"os"
+)
+
+var ErrCopySameFile = errors.New("src and dst must be different files")
+
+// CopyFile copies a file from src to dst.
+// When dst and src are the same file, returns ErrCopySameFile.
+func CopyFile(dst, src string) error {
+	if src == dst {
+		return ErrCopySameFile
+	}
+
+	// open the source
+	srcFile, err := os.Open(src)
+	if err != nil {
+		return err
+	}
+	defer srcFile.Close()
+
+	// stat it to get the mode!
+	srcStat, err := srcFile.Stat()
+	if err != nil {
+		return err
+	}
+
+	// open or create the destination
+	dstFile, err := os.OpenFile(dst, os.O_WRONLY|os.O_CREATE, srcStat.Mode())
+	if err != nil {
+		return err
+	}
+	defer dstFile.Close()
+
+	// and do the copy!
+	_, err = io.Copy(dstFile, srcFile)
+	return err
+}