[FIRRTL] Program Slicer for debugging

Nobody has claimed this yet.

We sometimes want to extract specific module(s) to debug the module. It is sometime tedious to make the module valid by hand if the module has many sub-instances. So it would be useful to have kind of program slicing tool which externalizes instances or memories at fir level.

Something like
```bash
./firrtl-slicer foo.fir -module-name="Bar" -subinstances={externalize, flatten-depth=n, flatten-all} -convert-abstract-reset-to={i1, async}
```
It might be fine just adding this feature to circt-reduce though. Also it might be possible to reuse ExtractTestCode logic.

Start by reading circt-reduce and the mentioned ExtractTestCode logic to determine where a slicer could fit. Clarify the scope and interface for selecting a module, externalizing or flattening subinstances, and converting abstract resets. Done should be demonstrated by coverage for the agreed slicing behaviors.

Written by the indexing model from the issue text.