Implement the required functionality for `updateChannel`

[andrewgordstewart]

https://www.notion.so/statechannels/Rust-SC-library-93e36a45aeb8495f8c0325e6423eb86b#813f990280c84797afc516eafdbf4ed7

[andrewgordstewart]

Blocking on #18 -- since there's a lot of overlapping work, we should be careful to assign two people to this work. I think pushUpdate is simpler, and should probably be done first.