Update Elements with new C code

[roconnor-blockstream]

After #326 it would be good to update the Elements code. It isn't critical since there isn't any change in the programming logic: just renaming of things, but it would be a good to do.

I'd like to task @delta1 with this, if he is willing to do so, since I think it would be good practice for someone else to try this process, and perhaps document it somewhere in the elements repo.

There is a two step process to perform. First update the C-master branch with a command like

git subtree split -P C  -b C-master

Which I run from a locally checked out copy of the Simplicity master branch. This should then be pushed to github for transparency reasons. It should always be fast-forward. If not, then something might be wrong.

The second step is to run

git subtree pull --prefix src/simplicity ~/Simplicity C-master --squash

in a locally checked out elements repo. Above I'm assuming that ~/Simplicity is your local Simplicity repo with an updated C-master branch. I suspect there is a way to point directly to the github repo instead here, and that might be better.

This command should commit an update to the src/simplicity directory. Then you can PR the change to the elements repo.

That should be it.

[delta1]

Hey @roconnor-blockstream thanks for the write-up, I will take a look at this

[delta1]

Created #330 to upstream C-master after subtree split.

Created ElementsProject/elements#1519 for the Elements update and doc.