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

androidmk/cmd/androidmk/androidmk.go

@@ -239,7 +239,8 @@ func convertFile(filename string, buffer *bytes.Buffer) (string, []error) {
 	}
 
 	// check for common supported but undesirable structures and clean them up
-	err := bpfix.FixTree(tree, bpfix.NewFixRequest().AddAll())
+	fixer := bpfix.NewFixer(tree)
+	tree, err := fixer.Fix(bpfix.NewFixRequest().AddAll())
 	if err != nil {
 		return "", []error{err}
 	}