Ability to get function-call expression from function body

It would be handy if it was possible to get function call expression from a given function body. E.g.
```python
call = func.calls['myfunc'] # return a CallExpr instance.
```

Search the framework for the function-body representation, the existing func.calls access, and the CallExpr class or related expression APIs. Determine how a call is identified by function name, then verify that the completed behavior returns a CallExpr instance for func.calls['myfunc']; the issue names no test file.

Written by the indexing model from the issue text.