Automating merging PRs from bindings for mmtk-core updates

[qinsoon]

For breaking changes in mmtk-core, we always make sure that the bindings we maintain work with the new changes. To achieve this, we will need to have pull requests in each binding, and they should be merged as soon as the PR in mmtk-core is merged. Currently we manually do this merging process. It would be good if we can automate this.

The automation should do the following:

1. We should specify which PRs/branches in the binding needs to be merged (possibly we can use the same specification as for the binding tests - there is no need to specify it twice).
2. We merge the mmtk-core PR.
3. We update each binding PR to refer to the latest mmtk-core commit (the merge commit in master)
4. We update Cargo.lock as the mmtk dependency is changed in the last step.
5. Commit and merge each binding PR.