avoid OrderBy().ToList() in DefaultContractResolver.CreateProperties

[SimonCropp]

Replace the LINQ ordering with an in-place List.Sort, and skip
sorting entirely when no property has an explicit Order. The previous
path allocated an OrderedEnumerable, a key-selector delegate, and an
intermediate buffer for every contract built.

Benchmark (net10.0, PropertyOrderBenchmark):

Count=5,   no order:  319 ns / 464 B  ->   63 ns /  96 B  (0.20x time, 0.21x alloc)
Count=20,  no order:  710 ns / 816 B  ->  106 ns / 216 B  (0.15x time, 0.26x alloc)
Count=100, no order: 4009 ns / 2736 B ->  388 ns / 856 B  (0.10x time, 0.31x alloc)
Count=100, mixed:    6201 ns / 2736 B -> 5570 ns / 856 B  (0.90x time, 0.31x alloc)