Provide propper AND / OR logic for `xs:pattern`

[Bergmann89]

#251 fixed the xs:pattern logic from beeing ANDed to being ORed, which fixes the issue of accepting multiple patterns, but it raises a new issue, since patterns across different derivation steps should be ANDed. A potential solution would be to store a Vec<Vec<String>> instead of just Vec<String> in the Constrains type.