Method to generate sector key

[magik6k]

There seems to be no good way to generate a sector key today - which is needed for snapdeals unsealing, and possibly for future unsealing service-type things.

One could call Precommit1 and Precommit2 with null bytes (PC1 alone doesn't generate the sector key - #1665)

[magik6k]

This is needed to enable filecoin-project/lotus#10316, and also to properly fix snapdeals unsealing in lotus

[vmx]

This is about getting the same bytes as TreeR, but without building TreeD or PC2. Only the layer building is needed.

[vmx]

There is now an API to run SDR only. It's exposed in the FFI at filecoin-project/filecoin-ffi#402.