[propose] Should we have event caller (f_back) in type_comments?

If you have multiple functions calling one function, it will look like this.

```python
{
"path": "~~~",
"line": 1,
"func_name": "called_func",
"type_comments": [
"(int) -> int"
"(int) -> str"
],
"samples": 2
},
```

I would like to know which function calls the `called_func` in this case.

```python
{
"path": "~~~",
"line": 1,
"func_name": "called_func",
"type_comments": [
{
"calling1_func": "(int) -> int",
}
{
"calling2_func": "(int) -> str",
}
],
"samples": 2
},
```

... because there are chances to have 10 or more samples (sadly).

## remarks

This must change the code below (and more. such as `FunctionKey`), but it maybe gets slower.

https://github.com/dropbox/pyannotate/blob/1e7ddf09fdbc6fa37b2cf261f36a5563a0fe772a/pyannotate_runtime/collect_types.py#L446-L448

So I want to know if this proposal is ideal or not. If ideal I think I can contribute when I have a time (maybe late of this month)