label graph with reasons and expansion strategies

Summary:
This diff adds labels to the edges and nodes in our graph.
- Edges are labeled with a *reason*, describing why a particular node was created from another node (i.e., which heuristic was used).
- Nodes are labeled with a *strategy*, determining how successors of this node should be created. Concretely, nodes in our graph now consist of a symbol (as before) and a strategy. The *strategy* of a node is derived from the *reasons* on the incoming edges.

The motivation is the following: For some heuristics I'm planning to implement, it's not enough to just know that we want to determine the successor nodes of, say, a particular class `C`. Instead, we need to know why we are interested in `C` in the first place to decide what successors to construct for `C`. A concrete example for this is implemented in the next diff in this stack, where a class can then be in our graph because it contains the definition of a root method *or* because it contains the definition of a type constant we care about.

It would be possible to do all of this with just *reason* values. We could enforce that each node only has incoming edges labeled with the same *reason* and use that to determine how to build successors. But for most reasons, there isn't actually a difference. Therefore, *strategies* effectively act as an equivalence relation on *reasons*, where having equivalent reasons implies having the same successor nodes in the graph.

Eventually, `reasons` will also be used for provenance tracking, and I want to be able to dump the graph (including reasons) to create .exp tests. But that's coming in the future.

Implementation note: I went back and forth between several designs for how to type the reasons and strategies. It would be nice to have some additional static guarantees (e.g., a node for a `Method` symbol can only have a `method` reason), but this ended up being pretty unwieldy. I a

Reviewed By: madgen

Differential Revision: D90081785

fbshipit-source-id: 3d9c8f8c5d8ee8316378d91607dc6ef3e905ba99

Author: frank-emrich