We were building essentially a CP of all control flow nodes:
```
Tuple counts for Essa::AssignmentDefinition::getValue#dispred#f0820431#ff/2@dd1f67vl after 2m45s:
733365 ~6% {3} r1 = JOIN Essa::TEssaNodeDefinition#24e22a14#ffff_30#join_rhs WITH Essa::EssaNodeDefinition::getDefiningNode#dispred#f0820431#ff ON FIRST 1 OUTPUT Lhs.1, Rhs.1, Rhs.0
376588 ~0% {2} r2 = JOIN r1 WITH SsaDefinitions::SsaSource::assignment_definition#9197156e#fff ON FIRST 2 OUTPUT Lhs.2 'this', Rhs.2 'result'
376588 ~0% {3} r3 = JOIN r2 WITH Essa::TEssaNodeDefinition#24e22a14#ffff_30#join_rhs ON FIRST 1 OUTPUT Rhs.1, Lhs.0 'this', Lhs.1 'result'
6965593033 ~2% {3} r4 = JOIN r3 WITH project#SsaDefinitions::SsaSource::assignment_definition#9197156e ON FIRST 1 OUTPUT Lhs.1 'this', Rhs.1, Lhs.2 'result'
376588 ~0% {2} r5 = JOIN r4 WITH Essa::EssaNodeDefinition::getDefiningNode#dispred#f0820431#ff ON FIRST 2 OUTPUT Lhs.0 'this', Lhs.2 'result'
return r5
```
We first tried preventing the join on `result`, but this caused the
characteristic predicate to blow up instead. Finally, we figured just
putting the `value` part in a field would be sufficient, and this did
the trick.
Seems like all other languages use a file called `DataFlowDispatch`. I
want to introduce a setup where we have (old) points-to based approach
in one file, and can develop a type-tracking based approach in another
file, so that's the reason for the naming differing slightly.
For which predicates go in which files, I have taken mostly inspiration
from C# and Ruby.
The output from running the test script is:
```
'rootdir/bar'
[13/Apr/2022 09:20:36] "POST /app/file-test/ HTTP/1.1" 200 2
'rootdir/bar'
[13/Apr/2022 09:20:36] "POST /app/file-test/ HTTP/1.1" 200 2
'rootdir/foo%2fbar'
[13/Apr/2022 09:20:36] "POST /app/file-test/ HTTP/1.1" 200 2
'rootdir/%2e%2e%2fbar'
[13/Apr/2022 09:20:36] "POST /app/file-test/ HTTP/1.1" 200 2
'rootdir/foo%c0%afbar'
[13/Apr/2022 09:20:36] "POST /app/file-test/ HTTP/1.1" 200 2
```
I didn't add a `.py` extension, so it wasn't extracted, since we don't
actually care about what we model in that file.
