List.contains - change in order to inline + typespecialize

[T-Gro]

Credit to @jindraiva , implements and tests #15720 .

Benchmarks showing original issue with current List.contains impl: https://github.com/jindraivanek/Benchmarks-list-contains/blob/master/Benchmarks/BenchmarkDotNet.Artifacts/results/ListBenchmarks.ListTests-report-github.md

(the here implemented option is called outerContainsWhatIf in the benchmarks - lowest allocation and fastest speed across the variants)

Basic example of how the lifting of equality check (=) from the inner rec function to the outer one makes inlining possible.

https://sharplab.io/#v2:DYLgZgzgPsCmAuACAlgO2G2iD2BXesATgMLarwCGaEiAbhcLlhHoQMZYC8AsAFCIDEcJIkKw2KVKiKlyVVDSwAPCAEZEPfoO0BbCvDYALRCvUB3ZPEN9t2qIgDaAXUQBaAHyIwDCLBu2Be0N1EBBEeHUPRC5EYMQoezRpEjJKamjw1T5/ASSZVPkaekZmVg4cxD5hSQxpHHx8uWoAdUN9AEkwOgYmRBZcdi4K6rRarGQIAFl9I2i4HQ052AXObpLhhAExCTyUpoVWjq7lNQ0K3RnjU0QLK3PBe2c3T29gX3vA2JCwiOeUKcuX3iiSkjTSBza8E6GQi2S0uVBe3BEEOUK6xV6/UGFRxvGqYgguGASFWeAISMKiFQFFQjmpqAA3ABWAB0AAYGfSXFVNgSifBUdDSQ0KS1IdD6XSacz2ZyaS4gA

[T-Gro]

This is ready now.