MachLinkEditData: Implement .ToString(), keep track of parents

[qmfrederik]

Make debugging a bit easier by:

• Keeping track of the parent of MachLinkEditData objects
• Implement a .ToString() which returns the start and end of the range, as well as the length

[filipnavara]

(Sorry for delayed response. Unfortunately I will have very limited internet access until tomorrow evening, and then I will have to deal with couple of medical issues as I return back to Prague. I will try to review remaining PRs until the end of the week.)

I tried to avoid the back-/parent links in the design to avoid some object cycles. It looks like with the current design it would not cause any cycles that could result in high memory usage but I will need to take another look at it to see if it can be done in a way that prevents any programming error or stray references.

[qmfrederik]

@filipnavara No worries, I was also travelling this week. I understand the concern about back references. This was mainly for debugging purposes; I think it's fine if you decide not to take the back references.