It's time. sd-json was already done earlier in this cycle, let's now
make sd-varlink public too.
This is mostly just a search/replace job of epical proportions.
I left some functions internal (mostly IDL handling), and I turned some
static inline calls into regular calls.
Let's define this in the generic interface and then import it into the
Monitor interface too.
This is preparation for adding an interface to resolve arbitrary RRs via
Varlink, which means we want the type in both interfaces.