Make Guile-GI MT-safe

[LordYuuma]

When writing C code for use with libguile, you should try to make it robust.

Currently, none of our functions are in any way MT-safe, at least not the large user-visible ones. Of course, so are many of the bindings we create, but there is nothing we can do about that. Still, as far as the library itself is concerned, we should use mutexes or RW locks to not impose unnecessary restrictions.