Add removal of gas-limited constraints to gas-savings proposal

[rvagg]

Ref: #1094
Ref: filecoin-project/builtin-actors#1586
Ref: filecoin-project/builtin-actors#1268
Ref: filecoin-project/lotus#10612

[rvagg]

Since this is a PR to a PR it doesn't need full approval and review by editors, but it does need approval from @irenegia, @AxCortesCubero & @zixuanzh to mess with their FIP doc.

But of course if anyone else wants to suggest edits, raise objections or suggest that this will decrease the likelihood of the original FIP being accepted, please speak up!

[jsoares]

Minor editorial comments. I don't otherwise see anything that warrants comments at this intermediate stage; the changes don't strike me as negatively impacting potential acceptance.

[anorth]

Happy to merge when @irenegia approves