Fix TODO comments throughout the solution

[LSViana]

This list is not final and will be accumulated as items are solved:

• [x] Make it easier to connect OutputNative size with pointer size in MapEntryNative
• [x] Remove mentions about *Native counterpart for event classes that don't need it
• [x] Fix tests for XmlElement.Observe() that weren't using a root-level XmlElement but should be
• [x] Fix tests for XmlElement.Unobserve() that weren't using a root-level XmlElement but should be
• [x] Fix tests for XmlElement.Parent() that weren't returning the correct value
• [x] Fix tests for XmlElement.String() that weren't testing for child XmlText nodes but should be
• [x] Remove mentions about read-only transactions on writeable functions (they will be tested later, after exception handling is added)
• [x] Remove mention to test Transaction.Dispose()
• [x] Remove comment from Output.Object
• [x] Remove comment from Map.Length()
• [x] Remove comment from TextChannel
• [x] Remove comment for Doc.Guid that will not be fixed because the GUID format isn't compatible with C#
• [x] Remove comment from InputNative because the size of the data can't be reduced or data will be lost
• [x] Fix the constructor of Output to support null pointer
  • [x] Update usages of new Output() to use ReferenceAccessor.Access(Output)
• [x] Implement test of getting null value from Map after fixing the Output constructor
• [x] Replace NotImplemented with NotSupported exception on the EventBranch constructor since all cases are now covered
• [x] Implement remaining tests marked as [Ignore] because they were waiting for implementations
• [ ] Implement IEnumerator<T>.Reset() on the following classes:
  • [ ] ArrayEnumerator
  • [ ] MapEnumerator
  • [ ] XmlAttributeEnumerator
  • [ ] XmlTreeWalkerEnumerator
  • [ ] Remove the documentation mention about these classes only supporting one-time enumeration
• [ ] Wrap the XmlElement with an XmlFragment before returning the value.
• [ ] Wrap the XmlText with an XmlFragment before returning the value.
• [ ] Dispose EventSubscription with Unsubscribe().

[SebastianStehle]

@LSViana We have this TODO in the code:

    // TODO This is a temporary solution to track the amount of transactions a document has open.
    // It's fragile because a cloned instance of the same document won't be synchronized and if a `Transaction`
    // is opened in the cloned instance, it'll receive `null` from the Rust side and will cause the
    // `ThrowHelper.PendingTransaction()` to run (which is acceptable since it's a managed exception).
    internal void NotifyTransactionStarted()
    {
        openTransactions++;
    }

    internal void NotifyTransactionClosed()
    {
        openTransactions--;
    }

Do you think we will ever have a final solution or shall we just remove this and also close this issue?

[LSViana]

@SebastianStehle I think we'll have some solutions if we ever look into that in detail but I didn't do it yet.

Given that, I'd just leave it there. It's not harmful and informs other developers who read the same code which is a positive detail.