Alias to `System.Collection.Immutable` types where appropriate

[xperiandri]

ImmutableHashSet and ImmutableDictionary can definitely be used in place of Set and HashMap Maybe ImmutableStack can work instead of List when collection initializers implemented?

[vzarytovskii]

Many of those have different semantics to how builtin collections currently behave, so just replacing them might not be a good idea. Needs to be accessed and tested carefully case by case.