Add more complicated Enumerable w/ Enumerable tests

[tgharold]

Add a test which has a List which contains elements which also have a List of string.

From what I can tell, there's no way for a ValidationAttribute validator to report back on all of the elements that failed validation within a simple IEnumerable<string> list.

At best, you might be able to write a ValidationAttribute validator which reports back on the index of the first failure within the list.

[tgharold]

Benchmark

|                                            Method |      Mean |     Error |    StdDev |    Median |   Gen0 |   Gen1 | Allocated |
|-------------------------------------------------- |----------:|----------:|----------:|----------:|-------:|-------:|----------:|
|  EnumerableExample_PassesAllValidation_NoChildren | 12.276 us | 0.4016 us | 1.1196 us | 12.130 us | 1.0681 | 0.0153 |   4.96 KB |
| EnumerableExample_PassesAllValidation_AllChildren | 81.965 us | 1.8714 us | 5.4294 us | 79.941 us | 7.8125 |      - |   36.8 KB |
|              RecursionExample_PassesAllValidation | 12.197 us | 0.4984 us | 1.4139 us | 11.984 us | 1.1597 |      - |   5.39 KB |
|                 SimpleExample_PassesAllValidation |  7.292 us | 0.2170 us | 0.6295 us |  7.197 us | 0.7401 |      - |   3.41 KB |
|                             SimpleExample_AllNull |  9.198 us | 0.2794 us | 0.8016 us |  9.057 us | 0.8545 |      - |   3.93 KB |
|        SkippedChildrenExample_PassesAllValidation | 14.321 us | 0.3610 us | 1.0358 us | 14.082 us | 1.3123 |      - |   6.04 KB |
|                    SkippedChildrenExample_AllNull | 18.423 us | 0.3992 us | 1.1324 us | 18.129 us | 1.6174 | 0.0305 |   7.56 KB |