feat: phase2-cli merge command

[DrPeterVanNostrand]

The phase2-cli merge command merges large and small param files.

The final phase2 participant generates small-raw params, which are converted into non-raw using phase2 convert. The final small-nonraw params are then merged into the large initial params to produce the final MPCParameters file for a circuit.

[porcuquine]

Looks good.

[vmx]

Your PR description really explains nicely what it does, it would be great to have that text also in the commit message for future reference. BTW: if you put that into your commit message, it also automatically shows up as the description of the PR (in case you don't have much experience with re-writing Git commits, I'm happy to help/do it).

Things could be simplified a bit, though I wasn't sure whether this additional parsing was intentional or not. It was easier to do it, than explaining it in comments, hence I created a new PR: #1244

[DrPeterVanNostrand]

@vmx Thanks, I added the PR description to the commit message.

PR: #1244

Yup, you're right. There is dead code.

I have one more phase2-cli pr for a command phase2 parse <path>, which just parses a params file into FileInfo and prints it. Which I've been using to verify that small and large params have the same values for delta_g1/g2, h/l_len, h/l_first, h/l_last

[vmx]

@DrPeterVanNostrand This PR needs a rebase. I guess the rest of it is then ready to go (i'd merge this one first, so that you don't have to rebase yet another time, i'll rebase my "split-keys" PR after this on is merged).