Only have one type of callable, but have an extra type of call.
A constructor call directs to an init callable
(should also handle `call` overrides at some point).
Removes steps from `ModuleVariableNode`s from `essaFlowStep`, and
instead puts them only in `jumpStep`. This cleans up the logic a bit.
This slightly broke the type tracker implementation (as it relied on
`essaFlowStep` being fairly liberal), so I have rewritten it to
explicitly rely on just familiar predicates for local and jump steps.
Additionally, we disallow Essa-to-Essa steps where exactly one of the
two nodes corresponds to a global variable (i.e. only local-local and
global-global steps).
Before:
```
[2020-09-18 14:03:57] (2587s) Tuple counts for RuntimeChecksBypass::uncheckedWrite#bbf#antijoin_rhs#1:
1270 ~8% {2} r1 = SCAN RuntimeChecksBypass::uncheckedWrite#bbf#shared AS I OUTPUT I.<1>, I.<0>
188197390 ~0% {3} r2 = JOIN r1 WITH #Callable::Callable::calls_dispred#bfPlus AS R ON FIRST 1 OUTPUT R.<1>, r1.<1>, r1.<0>
2425784042 ~1% {3} r3 = JOIN r2 WITH Expr::Expr::getEnclosingCallable_dispred#ff_10#join_rhs AS R ON FIRST 1 OUTPUT r2.<1>, R.<1>, r2.<2>
58 ~9% {2} r4 = JOIN r3 WITH project#RuntimeChecksBypass::checkedWrite#bfff AS R ON FIRST 2 OUTPUT r3.<0>, r3.<2>
return r4
```
After:
```
[2020-09-18 14:08:48] (5s) Tuple counts for RuntimeChecksBypass::uncheckedWrite#fff#antijoin_rhs:
24704473 ~2% {2} r1 = SCAN DataFlowPublic::localExprFlow#ff AS I OUTPUT I.<1>, I.<0>
23784154 ~6% {4} r2 = JOIN r1 WITH Expr::Expr::getEnclosingCallable_dispred#ff AS R ON FIRST 1 OUTPUT r1.<1>, 28, R.<0>, R.<1>
201391 ~2% {2} r3 = JOIN r2 WITH expressions AS R ON FIRST 2 OUTPUT r2.<2>, r2.<3>
23784154 ~0% {3} r4 = JOIN r1 WITH Expr::Expr::getEnclosingCallable_dispred#ff AS R ON FIRST 1 OUTPUT r1.<1>, R.<0>, R.<1>
1065242 ~20% {2} r5 = JOIN r4 WITH expr_value AS R ON FIRST 1 OUTPUT r4.<1>, r4.<2>
1266633 ~16% {2} r6 = r3 \/ r5
return r6
```
