While researching an observed performance issue with wildcard
queries, it was observed that the LiteralPrefix() method on
the regexp.Regexp struct did not always behave as expected.
In particular, when the pattern starts with ^, AND involves
some backtracking, the LiteralPrefix() seems to always be the
empty string.
The side-effect of this is that we rely on having a helpful
prefix, to reduce the number of terms in the term dictionary
that need to be visited.
This change now makes the searcher enforce start/end on the term
directly, by using FindStringIndex() instead of Match().
Next, we also modified WildcardQuery and RegexpQuery to no
longer include the ^ and $ modifiers.
Documentation was also udpated to instruct users that they should
not include the ^ and $ modifiers in their patterns.
This is a change in search result behavior in that location
information is no longer provided by default with search results.
Although this looks like a wide-ranging change, it's mostly a
mechanical replacement of the explain bool flag with a new
search.SearcherOptions struct, which holds both the Explain bool flag
and the IncludeTermVectors bool flag.
