t/40-leak.t: add a test. cyclic ref should be released when a watcher is released by $selector->cancel() method.
t/40-leak.t: a watcher should be free of cyclic ref even if the watcher is included in the callback closure.
t/40-leak.t: more memory_cycle_ok()s...
Selector, Watcher: watcher callback is now kept in Selector object along with watchers. This reduces the risk of cyclic reference further. Now watchers are free of cyclic refs even if they are included in their watcher callback closures. More cyclic refs are checked in the tests.
Change camelCase to snake_case. The affected methods are all private.
CPAN release v1.03