Generate a callsite identifier associated with each callsite

1. Generate a unique string name for a callsite that appears in every node
2. This identifier will refer to a single callsite and is used for trace data retrieval

@ke-yu
@lukechurch
@Steell
@Benglin

Ouch. That's a pretty bad bug, it's going to be horribly difficult to find. Please submit with a repro test case.

Critical next sprint.