Checks for sequences

[jackfirth]

It would be nice to have some custom checks that simplify assertions made on sequences.

• (check-contains? seq expected ...) For each expected, assert that seq contains at least one value that is equal to the expected. Maybe include a keyword argument to enforce order.
• (check-contains-exactly? seq expected ...) Like check-contains?, but if seq has any extra values then the assertion is failed.
• (check-has-size? seq expected) Same as (check-equal? (sequence-length seq) expected).

This would also help teach people that the "actual" value goes on the left, and the "expected" value(s) go on the right.