Fix and simplify DirectoryEntry validation

[jeremy-visionaid]

Ensure both the root and left sibilings are considered when checking for duplicate SIDs.

Also, remove redundant lookups.

[jeremy-visionaid]

@ironfede Looks to me like the root should also optionally throw if ValidationExceptionEnabled is set? i.e.

        private void LoadChildren(RBTree bst, IDirectoryEntry de)
        {
            if (de.Child != DirectoryEntry.NOSTREAM)
            {
                IDirectoryEntry child = directoryEntries[de.Child];
                // if (child.StgType != StgType.StgInvalid) // Should actually throw?
                {
                    List<int> levelSIDs = new List<int>();
                    LoadChildren(bst, child, levelSIDs);
                }
            }
        }

WDYT?

[ironfede]

@jeremy-visionaid I think thats named used in methods are not 100% representative or their function. For example SiblingIsValid seems to imply it checks for "validity" but it really checks for "existance" of sibling AND do validation. So I think it should be renamed "HasSiblings" and internally throws for validity exception.

[ironfede]

@ironfede Looks to me like the root should also optionally throw if ValidationExceptionEnabled is set? i.e.

        private void LoadChildren(RBTree bst, IDirectoryEntry de)
        {
            if (de.Child != DirectoryEntry.NOSTREAM)
            {
                IDirectoryEntry child = directoryEntries[de.Child];
                // if (child.StgType != StgType.StgInvalid) // Should actually throw?
                {
                    List<int> levelSIDs = new List<int>();
                    LoadChildren(bst, child, levelSIDs);
                }
            }
        }

WDYT?

I think it should throw.

[jeremy-visionaid]

@jeremy-visionaid I think thats named used in methods are not 100% representative or their function. For example SiblingIsValid seems to imply it checks for "validity" but it really checks for "existance" of sibling AND do validation. So I think it should be renamed "HasSiblings" and internally throws for validity exception.

Hmm, I might take it further and say that the method actually isn't even aware of sibilings - only the caller is, so "HasSiblings" is also arguably misleading. I think perhaps I could just rewrite it to avoid having a boolean return at all.

[jeremy-visionaid]

@ironfede I've rewriten to avoid having a bool method, and added another commit on top that fixes the case where an exception was not thrown if the root entry was invalid too.