Support cancellation of timer operations

[civodul]

This patch series fixes #109 by supporting a "cancel" function for base operations and using it to remove timer wheel entries when a timer "loses" in a choice operation.

I'd like feedback in particular on the last commit: can someone confirm set! good enough, or should we use an atomic box instead?

I ran the test suite of the Shepherd and that of Cuirass against this branch: the former has good coverage though it uses a single POSIX thread, the latter has not-so-good coverage but uses multiple POSIX threads. Both passed.

Thoughts?