FILTER isImmutable

[rogerlucena]

This PR comes to add support for the isImmutable FILTER function in BadWolf, as detailed in #129. It also comes to help resolving what was requested by #115.

To illustrate its implementation on the driver side, this PR also adds support for this FILTER in the volatile OSS driver (in memory.go).

Now, the user can write queries like:

SELECT ?p, ?o
FROM ?test
WHERE {
    /u<peter> ?p ?o .
    FILTER isImmutable(?p)
};

That would return only rows for which the predicate ?p is not temporal. For other examples of queries have a look at the newly added tests in planner_test.go.

This FILTER function also works for object bindings in the clause, along with predicate aliases and object aliases too.

[thiagovas]

Can you add some tests in memory_test.go as well?

[rogerlucena]

Can you add some tests in memory_test.go as well?

In the second to last commit I have already added a total of 22 new tests in memory_test.go too: 2 for each one of the 11 possible calls to the driver (one test for FILTER isImmutable predicate field and one for FILTER isImmutable object field for each possible call).

[rbkloss]

Could you change the name of the test struct a bit?
You could explain what is the want, e.g.,
want -> ExpectedCountByObjectName.
s -> query_subject
p-> query_predicate

Also the logic in the test body seems a bit too complex for tests.
Instead of decrementing the map counter and deleting if the count gets to 0,
WDYT of building the "got" map and calling equal on both?

instead of:

for s := range ss {
  sStr := s.String()
  if _, ok := entry.want[sStr]; !ok {

  t.Fatalf("g.Subjects(%s, %s, %s) retrieved unexpected %s", p, o, entry.lo, sStr)						 
  }
  entry.want[sStr] = entry.want[sStr] - 1
  if entry.want[sStr] == 0 {
    delete(entry.want, sStr)
  }
}

something like:

got := map[string]int{}
for o := range os {
  oStr := s.String()
  got[oStr] += 1
}
if !cmp.Equals(entry.want, got){
  t.Fatalf("g.Subjects(%s, %s, %s) retrieved unexpected %s", p, o, entry.lo, sStr)
}

[rogerlucena]

As we commented through chat, I can be doing this in a separate PR after this PR and the PR 154 are merged. Also, I will be letting this task as P2 for now to focus on current tasks, as we commented through chat as well.