support usdt and dtrace

[timotheecour]

refs: nim-lang#14559 (comment)

we could start offering dtrace-specific apis to make it easier to use in nim; it's an incredibly powerful tool for debugging (performance, counters, logic etc). Allows tracing running applications not written with dtrace in mind, or custom tracing of applications with dtrace hooks that can be changed at runtime.

All with zero overhead if you're not tracing, so it's easy to switch on/off. As an example, nim binary itself could be instrumented to have counters for various things (number of allocations, number of calls to key functions, vm calls etc)

the D language (not the dlang one ;-)) could potentially be generated from nim or some nim DSL too.