Consider switching to reference-based value comparer when a primitive collection is immutable/read-only

[roji]

Our default value comparer(s) for primitive collections do deep comparison, i.e. they check equality for each and every element. When we implement nested primitive collections (#30713), this will have to be recursive.

If the primitive collection type is known to be immutable (e.g. implements IImmutableList), we can instead switch to the much cheaper reference comparison instead.

One possible objection here is that the immutable list may contain mutable elements, in which case the value comparer would miss something being changed (IIRC we discussed this and decided its OK).

[ajcvickers]

Note we can do this safely if we can determine that the elements really are immutable, which could be related to general work to support immutable types.