slices: make slice equality comparator flexible on target type

[shoenig]

This PR enables SliceContainsFunc to accept a different type as the
target from the slice elements type. For example if we have a slice
of Person objects but want to detect if the slice contains a person
with a particular name (string), we can now use a comparator of
signature func(Person, string) bool - rather than needing a whole
Person struct for the target.

Closes #88

Worth noting that technically just the predicate function is sufficient, as is done in the slices package https://pkg.go.dev/golang.org/x/exp/slices#ContainsFunc - however in the context of test case assertions I think it's more valuable to have the target value explicitly part of the assertion signature.