Switch CI to GitHub Actions (#48)

Co-authored-by: Fabrizio Ferrai <fabrizio.ferrai@gmail.com>

Bump version number (#33)

Consider something a builtin when there's no `sourceSpan` (#32)

* Consider something a builtin when there's no sourceSpan

* Refactor `extractPackageName`

* Use module name to check if a declaration is built-in

* Recognize `Prim` module as built-in; add tests

* Code style

Fix #31

Co-authored-by: Vladimir Kalnitsky <klntsky@gmail.com>