fix: revert backtrace frame from entrypoint to method

[Stebalien]

This is a pretty public API (information gets displayed to the user) and I'd rather not go about changing it at the moment. We'll probably bring this back eventually, but I'm trying to cut a release and integrate with the FFI with minimal outward-facing changes.

[Stebalien]

We'll probably bring this back eventually

Also, when we do that, I expect we'll change the Frame to make more sense in this case in this case, maybe? Maybe even include the code CID?

[codecov-commenter]

Codecov Report

Merging #1956 (ea0319d) into master (6722164) will decrease coverage by 0.01%.
The diff coverage is 100.00%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1956      +/-   ##
==========================================
- Coverage   75.78%   75.77%   -0.01%     
==========================================
  Files         157      157              
  Lines       15447    15449       +2     
==========================================
  Hits        11706    11706              
- Misses       3741     3743       +2     

Files	Coverage Δ
fvm/src/call_manager/backtrace.rs	84.05% <100.00%> (ø)
fvm/src/call_manager/default.rs	89.92% <100.00%> (+0.02%)	⬆️

... and 2 files with indirect coverage changes

[Stebalien]

Merging now so I can start cutting releases.