• Added explicit boolean overloading for trace objects. Without this, Perl
  will use the object's string overloading and then check the truthiness of
  the returned string, which is a lot of extra work. This can produce
  significant slowdowns in some cases, as seen in
  https://github.com/plack/Plack/pull/697. Requested by Tatsuhiko
  Miyagawa. GH #23.