Python: Prevent bad joins

Adds a few unbinds to prevent bad joins from occurring.

Firstly, we never want to join `StepSummary::step` with
`TypeTracker::append` on `summary` as the first join, as the resulting
relation is absolutely massive. So we decouple the two occurrences of
`summary` by unbinding each of them.

Secondly, in some cases the node we're stepping to (`nodeTo` for type
trackers, `nodeFrom` for type backtrackers) will get joined eagerly
with the typetracker one is defining, and again this produces an
uncomfortably large intermediate join. A bit of unbinding prevents this
as well.

Author: tausbn

python/ql/src/experimental/typetracking/TypeTracker.qll

@@ -227,8 +227,8 @@ class TypeTracker extends TTypeTracker {
   pragma[inline]
   TypeTracker step(LocalSourceNode nodeFrom, LocalSourceNode nodeTo) {
     exists(StepSummary summary |
-      StepSummary::step(nodeFrom, nodeTo, summary) and
-      result = this.append(summary)
+      StepSummary::step(nodeFrom, pragma[only_bind_out](nodeTo), pragma[only_bind_into](summary)) and
+      result = this.append(pragma[only_bind_into](summary))
     )
   }
 
@@ -370,8 +370,8 @@ class TypeBackTracker extends TTypeBackTracker {
   pragma[inline]
   TypeBackTracker step(LocalSourceNode nodeFrom, LocalSourceNode nodeTo) {
     exists(StepSummary summary |
-      StepSummary::step(nodeFrom, nodeTo, summary) and
-      this = result.prepend(summary)
+      StepSummary::step(pragma[only_bind_out](nodeFrom), nodeTo, pragma[only_bind_into](summary)) and
+      this = result.prepend(pragma[only_bind_into](summary))
     )
   }