move this package into the statechannels monorepo

[geoknee]

For:

• we use a monorepo; there's little point unless everything (or almost everything) lives inside
• the usual upsides of a monorepo, including
  • we can make atomic commits across this package and (say) nitro-protocol
• we can leverage existing publishing workflows instead of doing it manually as we have been doing

Against:

• the usual downsides of monorepo, including
  • monorepo build time will increase (a bit)
• it will be (a bit) more difficult for external folks to discover/understand this package if it is buried in a monorepo
• it is more difficult to ensure the published package is not broken on npm (e.g. missing dependencies)
• BUT... we don't yet have any external consumers of this package

[geoknee]

(Another alternative is to absorb the code into nitro-protocol itself. But that might be a step too far?)