[CSBindings] Refactor getPotentialBindings into inferBindingsFor + infer

Split `getPotentialBindings` in two:

- `inferBindingsFor` should take responsibility for binding constraints
  to be evaluated as potential binding sources;
- `PotentialBindings::infer` would process constraints provided by
  `inferBindingsFor` and "extract" bindings from them.

This is done to separate "sourcing" logic from "inference" and
make the former pluggable, so bindings could be computed based
on changes in constraint graph in the future.

Author: xedin