Introduce "scripting" domains

Introduce "scripting" domains

The Python and Guile code exposed the internal domain constants both
as attributes of symbols and as values to pass to lookup functions.

Now, perfect backward compatibility here can't be achieved: some
symbols are going to have domain changes by the end of this series.
However, it seemed to me that we can preserve lookups using the basic
domain values.

This patch implements this by exporting the "or"-able search constants
with an extra bit set.  Then it introduces some functions to convert
such constants to domain_search_flags.  This will be used by the
Python and Guile code, so that both old- and new-style lookups will
work properly; and while preserving the idea that the domain constants
can be compared to a symbol's domain.