Change FromRawTracepointArgs::arg to return T rather than *const T which
seems to have been returning a dangling pointer.
Arguably this is not strictly necessary; edition 2024 seems to be
focused on increased strictness around unsafe code which doesn't unlock
new functionality for our users. That said, this work revealed an
apparent bug (see above) that we wouldn't otherwise catch due to
allow-by-default lints.
Added logic in expand function in both kprobe.rs and uprobe.rs for valid
macros. Now, kprobe & uprobe proc macros only accept ProbeContext, and
kretprobe & uretprobe only accept RetProbeContext.
Ref: #700