Provide an `arg()` method in `RawTracepointArgs` wrapper of
`bpf_raw_tracepoint_args` and also in `RawTracepointContext`, so
it's directly available in raw tracepoint programs.
The methods and traits implemented here are unsafe. There is no
way to reliably check the number of available arguments, so
requesting a non-existing one leads to undefined behavior.
The const-assert crate doesn't even compile with stable rust, so we
shouldn't depend on it. Instead we replicate its functionality behind
cfg(unstable) which is set at build time based on the toolchain in use.
Tamir Duberstein
Mike Rostecki
Wouter Dullaert
Dave Tucker