This PR:
- Closes#1647
It gives compilation errors for language features that require more
substantial support (recursion, polymorphism). The additional features
are to be implemented in future separate PRs.
* Adds a new target `geb` to the CLI command `juvix dev core compile`,
which produces a `*.geb` output file in the `.juvix-build` directory.
* Adds a few tests. These are not yet checked automatically because
there is no GEB evaluator; checking the `*.geb` output would be too
brittle.
