Option `contains_value_matching` and related assertions

[florian1345]

Add assertions for Option-values to check whether they contain a value matching a given predicate (or not). Suggested variants:

• contains_value_matching: Passes if value is Some(x) and x matches the predicate.
• contains_value_not_matching: Passes if value is Some(x) and x does not match the predicate.
• does_not_contain_value_matching: Passes if value is None or Some(x) and x does not match the predicate.