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

We have get the value from dictionary (in callInstance), so should this line be
```functionCallInstance = ++callInstance;```? Should CallsiteGuidMap be updated later on?