Only include xapian.h in one place, so we can have consistent options.
With that in place, we can enable C++ move semantics.
We don't do anything with that yet, but we check in the meson.build file
to see if we have the required xapian version.
Implement a new query parser; the results should be very similar to the
old one, but it adds an Sexp middle-representation, so users can see how
a query is interpreted.
