add publication workflow using cargo workspaces

[nicbus]

resuming from PR #143, here is a proposal to handle publishing via cargo workspaces

requirements

• the cargo login token configured in github as a secret named CRATES_IO_TOKEN

proposed release steps

• prepare the master branch for the release
• create or update the release branch to be aligned with master
• create a release commit (typically with the updated changelog) on the release branch, including a line that ends with "new version <version>" in the commit message (see example below)
• push the release commit, triggering the workflow

what the workflow will do

• checkout the release branch
• get the new version from the commit message (e.g. 0.2.0-rc.2 in the example)
• update crate versions and inter-dependencies (using exact versions for dependencies, with the = symbol)
• commit and tag the version change, the tag will be prefixed with a v (e.g. v0.2.0-rc.2)
• publish the packages to crates.io (using the token provided via github secrets)
• merge and push the new commits to the master branch

other considerations

• I started trying to trigger the workflow with a tag (e.g. rel0.2.0-rc.2) but it didn't work since a tag-triggered workflow can't push a new tag (e.g. v0.2.0-rc.2), as it's forbidden by github (to avoid workflow loops AFAIK)
• I also re-wrote the inter-dependencies in the form <package> = { path = "<path>", version = "=<version>" } to avoid cargo ws version errors on lnpbp_derive of the kind failed to select a version for the requirement
• the cargo ws version command produces some other errors of the same kind (see example below) but I left those as they are for now, since they appear related to the tokio/socket2 dependency issue (weren't there some days ago) and don't make cargo ws version or cargo ws publish fail

examples

commit message:

update changelog

 * new version 0.2.0-rc.2

cargo ws version error:

error: failed to select a version for the requirement `socket2 = "=0.3.15"`
candidate versions found which didn't match: 0.3.17, 0.3.16, 0.2.4, ...
location searched: crates.io index
required by package `lnpbp v0.2.0+2 (/home/dieeasy/git/lnp-bp/rust-lnpbp)`

conclusions

there surely are many improvements than can be done, like more and stricter checks to avoid release issues, but it should be a good starting point

let's discuss and optionally wait for a fix to #159 (and then document the final release procedure before merging)

[dr-orlovsky]

Solved by 9f5c179