ptrace: Ptrace{Requests, Events, Options} are not in an enum

[chaosagent]

https://github.com/nix-rust/nix/blob/master/src/sys/ptrace.rs#L14-L63

From CONVENTIONS.md:

We represent sets of constants that are intended as mutually exclusive arguments to parameters of functions by enumerations.

[kamalmarhubi]

Thanks for the report! Saw your #438 as well. Going to comment there.

[Susurrus]

I believe this is now resolved with #749.