add usdt probes

[cyphar]

bpftrace supports these, so we should add them. I guess a first pass would just add them for each top-level method, and we can then see if adding them elsewhere would be helpful.

With regards to registration, we should probably make this a feature and re-export usdt::register_probes (as suggested by the usdt crate documentation). However, for the C API this should probably be enabled by default and auto-registered using ctor.