Contract Result matchers

Expand `match_ok` and `match_err` to optionally receive a contract as the second argument:

```ruby
result = some_method

case result
when match_ok(:ok, Contract) then 'has status :ok and underlying data matches Contract'
when match_ok(:ok) then 'other results with status :ok'
when match_ok then 'successful results with status different to :ok'
end
```