Validate a struct collection that a property holds

[tgharold]

Motivation

The validator reads only the properties of reference types, so a property whose type is a struct was skipped. That included a struct collection such as ImmutableArray. The same struct was validated when it was an item of a list, since the nested collections change in #82. A model could therefore pass validation only because the collection sat one level higher, which is a false pass of the kind the 3.0 series is removing.

An independent review of everything since v2.3.3 found two bugs in the first version of this change. Both are fixed here, with specs that failed before the fix.

Summary

• A property of a struct type is walked when the struct is a collection of objects that can have attributes, and for a Nullable of such a struct. The path is the same as for a class collection, for example Lines[0].Sku. A struct that is not a collection, such as a Money, is still not walked.
• A struct collection that is its default value, such as an ImmutableArray nobody set, is skipped, as an item already was. The check now compares memory with RuntimeHelpers.Equals and no longer calls the Equals of the caller's struct. Before, a struct that compared only an Id passed while it held invalid objects, and an Equals that threw ended validation.
• The default skip applies to a property only when the property is declared as a struct. A property declared as an interface or object was always enumerated, and still is. This closes a false pass that the first version introduced.
• The branch includes a merge of master, so Add a maximum depth of 128 levels #83 (maximum depth) applies. A struct collection property is counted like any other collection, and a computed one now stops at the limit and fails the validation.
• README and CHANGELOG are updated. The "now throws when enumerated" and "never ends" cases are folded into the existing BREAKING enumeration entry. Stale comments and version labels (2.4.0 now 3.0, four checks now five) are fixed.

Not in this PR: the depth is still counted along the walk's route, so a valid graph with many back-references can fail at 128. That is a design problem in #83, and it gets its own PR (a breadth-first walk). KeyValuePair and tuple properties are also still not walked.

Test plan

• The first commit unskips 8 specs and adds specs that failed until the fix. The second test commit adds 12 more that failed until the fix to the default check.
• Guards for a struct collection against the maximum depth, for a dictionary value that is a struct collection, and for IsWalked and IsLeafType.
• Full suite on net8.0 and net10.0: 551 passed, 6 skipped, 0 failed.
• The net481 test project builds with -p:IncludeNetFramework=true.
• Windows CI runs the net481 tests.