Fix Geiser for Guile 3.0.6

Fixes #9 (closed)

In geiser-eval REPL meta-command: All mod, form and args are now syntax objects. The geiser-guile's logic will handle mod and form as is because they're just passed to guile's eval and compile procedures. args are processed by geiser-eval meta-command itself, so it's necessary to convert it back to a datum. We lose some metadata, but all elements in the args list are also syntax objects so I don't think it's a big deal.

syntax->datum was introduced before guile 2 so this change is backward compatible.

Of course I need to give credit to @flatwhatson and especially @wingo, who really helped finding what's wrong, and @jaor for patience.