Copyright years
Add tamer::preevent<> template.

A preevent<> is a prototype event<>: an event<> which has not yet been
instantiated into a heap-allocated object. Preevents are available
only for simple rendezvous and <= 1 slot argument. Preevents can
automatically turn into regular events through casting or construction
(but not template argument deduction, unfortunately, requiring some
additional overloads).

Preevents useful because they are much faster in the common case when
an event triggers immediately, rather than after any delay. The
test/t08 test creates 10,000,000 events, 1023/1024 of which are
triggered immediately. It runs ~80x faster using preevents than
events!

The issues with template argument deduction probably mean that we
should have a separate make_preevent() call. But screw it for now.

Signed-off-by: Eddie Kohler <ekohler@gmail.com>