v1.23-synth-test-4

Synthetic PoRep (#400) (#418)

* feat: add various new APIs for Lotus

Those new APIs are about:

 - decoding a range of an updated empty sector
 - generating TreeRLast
 - generating TreeC
 - running SDR

* docs: document the new APIs

Add API documentation to the new APIs. Also change one parameter name
for more consistency.

* wip Go bindings for new proving APIs
* Add nicer wrapper for GenerateSDR
* feat: update proofs to the latest release
* cleanup comments in proofs.go
* feat: update dependencies to WIP synthetic-porep code
* feat: add example of using SynthPoRep in existing test
* feat: wire in new APIs
* feat: add clear_layer_data API and more robustly use synth-porep in test
* feat: go-wrapper for ClearSyntheticProofs
* merged synth work
* Update deps after (messy) rebase
* missing v21 engine mapping
* newer cargo lock
* feat: point to fvm master after synthporep changes landed
* update the fvm & bls-signatures (to match the fvm)

---------

Co-authored-by: Volker Mische <[email protected]>
Co-authored-by: Łukasz Magiera <[email protected]>
Co-authored-by: Aayush <[email protected]>
Co-authored-by: Andrew Jackson (Ajax) <[email protected]>
Co-authored-by: Steven Allen <[email protected]>