lib/parse-sexp: 'starts-with' wildcard searches

lib/parse-sexp: 'starts-with' wildcard searches

The many tests potentially overkill, but they could catch typos in the
prefixes table. As a simplifying assumption, for now we assume a
single argument to the wildcard operator, as this matches the Xapian
semantics. The name 'starts-with' is chosen to emphasize the supported
case of wildcards in currrent (1.4.x) Xapian.