Make bpfix not modify the input tree

Make a new object called Fixer to hold the state of the tree, and
make a copy of the input tree so the original doesn't get modified.

Test: bpfix_test.go
Change-Id: I1dc6fd99158c8b0e1db029df99e6cf72699a5e63

bpfix/cmd/bpfix.go

@@ -75,7 +75,8 @@ func processFile(filename string, in io.Reader, out io.Writer, fixRequest bpfix.
 	}
 
 	// compute and apply any requested fixes
-	err = bpfix.FixTree(file, fixRequest)
+	fixer := bpfix.NewFixer(file)
+	file, err = fixer.Fix(fixRequest)
 	if err != nil {
 		return err
 	}