Improve build process for distributed coroutines

[chriso]

The build process needs improvement.

Users must install a separate tool — coroc — and then run it prior to calling go {run,build}. The coroc compiler mutates files (adding //go:build !durable), adds new ones throughout the tree (*_durable.go), and potentially vendors dependencies and/or the entire GOROOT in the local directory.

Ideally users could just run dispatch run or dispatch build and this would all happen automatically, and in a temporary directory so that the module isn't mutated.