Fix broken module context awareness in Guile 3.0.

In Guile 3.0.7 (and in 3.0.6, too), the result of finding the module for the current Scheme buffer does not result in a list of symbols like (foo), but rather a syntax object. Using syntax->datum in the ,geiser-eval metacommand performs the necessary transformation that makes calls to find-module work in procedures such as ge:compile in (geiser evaluation).

I also tested that this patch on Guile 2.2.7, which doesn't have this problem, and the syntax->datum call is a noop and everything continues to work as it should. I have not tested on Guile 2.0.