Split into multiple packages

[cljoly]

We have been considering splitting the crate into multiple packages for some time. This is a proposal to go ahead and split out the async-tokio-rusqlite feature into its own package (rusqlite_migration_tokio_async).

Pros

• Different stability guarantee per package: the async implementation is much less mature and so it may need to break compatibility to fix what in hindsight proves to be nonsense. I'm other words, while most of the library is ready for 1.x semver warranty but the async package isn't because it may require breaking API changes.
• Makes unit testing easier than using with the various features, since we currently have a separate crate for tests.
• We can go back on this decision: the package can be integrated as a feature, with some breaking API changes if necessary.

Cons

• Additional maintenance work: need to publish multiple packages
• Harder to use for users: additional package to install
• More complex git tags: how can we know what a git tag refers to? We can take the following convention:
  • Tags starting with “v” are for the main package (i.e. rusqlite_migration)
  • Tags starting with “rusqlite_migration_tokio_async-” are for package rusqlite_migration_tokio_async.
  • This convention is similar to the one used by the regex crate
• Why would the async feature be the only reason to create a separate package? Why not do it for the folder feature as well
  • These features are smaller and easier to get right
  • It’s ok if we did create many packages to test features, before removing them eventually. It does not create long term problem, besides the additional maintenance overhead

Proposal

We really need to get feedback on the async package and releasing alpha versions has not been enough to get users to come back with comments. With the difference in maturity between the async features and the rest of the code, it makes sense to create separate packages, where the async one could have more frequent releases and breaking changes if needed.

Let’s figure out how we could split into multiple package (in particular, how that would work with the from-directory feature).

[cljoly]

Started on this with #111

[cljoly]

Next step undertaken with #117. Unfortunately, it doesn't look like it can work without losing, for instance, a common error type for async and sync code. Not sure it's worth losing this.

[cljoly]

I ended up renaming the feature to have alpha in the name (#123) so that we can move forward with a new stable release. It does not mean that we won’t have a separate package,only that it won’t be right now.