Coalesce dependents onto single line in Typing_deps.dump_debug_deps
Summary:
This changes the backing store for the debug tracing in Typing_deps from a set of strings (each representing an edge) to a mapping from string (representing a dependency) to a set of strings (representing its dependents).
When printing, dependents are printed on a single line if they fit within 78 characters:
```
Class FooParent -> Class Bar, Class Foo, Class IFoo
```
And on multiple lines if they do not:
```
Class FooParent -> Class Bar,
Class Foo,
Class IFoo
```
Reviewed By: dabek
Differential Revision:
D10247323
fbshipit-source-id:
e60b7343e9a153dabb4f19f6d306f8ef471ffe2a