Use a faster contains expression for "start of word" matches.

For the special case of address completion, we are really only
interested in matching at the start of various parts of the name, first
name, email, etc. So for that case, use bif:contains('foo*') rather than
the much more expensive regexp match. Speeds up addressee completion
something fierce for a minor loss in functionality, namely arbitrary
substring matching.
(cherry picked from commit c37f8d7023c3e486762af0379e36708d7eb99cea)