Split `probe!` and `probe_lazy!` for arg eval

[cuviper]

We now evaluate probe! arguments unconditionally, while probe_lazy! avoids that when there's no observer. For SystemTap SDT, that means only the latter defines and uses a semaphore.

Closes #19.

[caizixian]

Thanks for the PR. It looks good. And lazy is an appropriate name as the arguments are computed on demand.