toolchain: Provide a tool for migrating repos & pallets across backwards-incompatible changes

[ethanjli]

Go has a program called gofix which can be used to migrate codebases to newer versions which involve backwards-incompatible changes. It would be nice if we could provide a similar tool to migrate packege repos and pallets from older Forklift versions to newer Forklift versions (when backwards-incompatible changes are made to the Forklift spec). However, I don't know how to do this in a reasonably simple and maintainable way; currently, I just manually make all the changes myself, which has been tolerable so far.