Report a struct once when a collection repeats it

[tgharold]

Motivation

A struct has no identity, so the walk reported an invalid struct once for each route that reached it. The extra routes came from two sources. First, a collection that is an item or the root object had its framework properties walked, and those repeat the items: a LinkedList<Line> item reported one struct four times, through [0], First.Value, First.ValueRef and First.List[0]. Second, a collection that two routes reach was enumerated once per route. The same collection held in a property already reported once, so an item and a property behaved differently. This is a 3.0 decision ("fix both causes"), because it changes the result list.

Summary

• For a collection, GetWalkedProperties leaves out the properties that System.* types declare, such as ArraySegment<T>.Array, LinkedList<T>.First and Dictionary<TKey, TValue>.Values. Properties that a user collection type adds, such as PagedList.Cursor, are still walked. IsLeafType uses the same method, so int[] and ArraySegment<int> become leaf types. The Array.SyncRoot special case in the walk is gone.
• The walk remembers each collection it enumerated. A second route enumerates it again, so an item that a Validate method added in the meantime is still found, but skips its struct items. Class items were already dropped by reference. A first version enumerated each collection only once, and it broke An_item_that_adds_to_the_list_that_holds_it_does_not_make_enumeration_throw, so it was narrowed.
• BREAKING: duplicate paths go away. An object that only a framework collection property reached is no longer validated: an array element outside an ArraySegment, or a dictionary's comparer. A jagged int[][] is no longer enumerated.
• Known limit that stays: two properties that each return a struct, such as a computed copy, give two values and two results.
• StructItemsReportedTwiceTests is now StructsReachedByTwoRoutesTests. README and changelog are updated.

Test plan

• Specs first: 9 new specs fail against master, and the guards pass
• With the fix: 617 passed, 7 skipped on net8.0 and net10.0
• Test project builds for net481 with -p:IncludeNetFramework=true
• CI passes, including net481 on Windows