Compile on stable Rust

[alexcrichton]

This removes the usage of #[fundamental] and intrinsics::type_name in the
notifications module. The Notifyable trait and Notifier struct were also
removed in favor of just using &Fn(Notification) where necessary. This change
is less ergonomic when crossing crate boundaries where Notifier naturally
implemented Notifyable for multiple notification types before. Instead, now
closures must be written as:

&|n| prev_handler(n.into())

Which is to say that when crossing crate boundaries you need to remap
notification closures and leverage a From implementation.

This crate does not currently compile on the stable channel due to the usage
of panic::catch_unwind in the curl crate for now, but that will get fixed with
the next stable release, this is just paving the way forward!

Closes #58

[Diggsey]

Nice! This isn't a criticism, but why did you choose to explicitly call .into() rather than making the existing ntfy!(x) macro expand to &|n| x(n.into())?

[alexcrichton]

Ah yeah that'd be possible, I just figured that the layer of a macro indirection probably wasn't worth it.

[bors]

☔️ The latest upstream changes (presumably #420) made this pull request unmergeable. Please resolve the merge conflicts.

[brson]

lgtm