New testing operator: `is-one-of`

[blerner]

Semantics:

foobar() is-one-of [set: val1, val2, val3]

passes if the result of foobar() is a member of the given set. This is useful occasionally, e.g. when dealing with functions that might return values in some arbitrary order: the set can specify the orderings that are anticipated. (Note that this is different from converting the result of foobar() into a set, and comparing it with an expected set for equality, because foobar() might return a value with duplicates inside it...)