Allow more elaborate patterns (like specifying suffix/prefix) for bindings inside HAVING clauses

It would be useful to be able to apply more ellaborate patterns for bindings inside the HAVING clauses, like being able to specify the suffix/prefix of a `TYPE` binding for example:

```
SELECT ?o, ?o_type
FROM ?test
WHERE {
?s ?p ?o TYPE ?o_type
}
HAVING ?o_type LIKE "/country/corp%"^^type:text;
```

With something like a `LIKE` keyword and a protected symbol such as `%` to represent that anything can appear at that position, as in the example above on which we specify a prefix for `?o_type`.

Given a graph containing hierarchical node TYPEs, such as `/country/corp` and `/country/corp/employee` for example, this feature would be especially handy.

To achieve that, it may be helpful to have a look at functions such as `strings.HasPrefix` as used for covariants in BadWolf here: https://github.com/google/badwolf/blob/dd89f9bf4852586b23d693d4313d8dad9d23f6f3/triple/node/node.go#L46 .