Switch to Project.toml and remove Nullable dependency

[alyst]

This package is required by GraphIO.jl, so I'm refreshing it to work on current Julia versions (1.x):

• replace Nullable{T} with Union{T, Nothing} and drop Nullable.jl dependency
• replace REQUIRE with Project.toml (using gen_project.jl) script

It also makes sense to set up @JuliaRegistrator and @JuliaTagBot for the repository.

[coveralls]

Coverage decreased (-8.2%) to 85.017% when pulling a80b50d on alyst:remove_nullable into 1b384cc on andrewcooke:master.

[alyst]

Bump :)
I might do a few other touches to this PR (in particular, to improve the coverage), but it would be nice to know whether the merging would be possible in principle.

[CarloLucibello]

Hi, I can merge your PR but probably I don't have permissions to activate registrator/tagbot and @andrewcooke is awol. We could merge this when you are ready and then fork if needed

[alyst]

I think it's ready for review. The coverage fell, but the tests didn't change. So it looks that's because the identification of coverable lines has changed.

[alyst]

I've rebased the branch against the current master, updated GitHub actions and switched to GitHub CI. Would be nice to merge :)