Implement TryFrom for Destination

[seanmonstar]

With TryFrom now stable, we can provide a TryFrom<Uri> for Destination. To prevent requiring the new Rust compiler, this can include a #[cfg] attribute that is set in the build.rs.

[M3rs]

I'd like to give this a shot. I think I have the implementation working, but I don't know a lot about #[cfg] in built.rs.

[seanmonstar]

Similar to how it's currently checking for Rust 1.30, and if found, printing cargo:rustc-cfg=error_source, a new check can be added for Rust 1.34, and if found, print cargo:rustc-cfg=try_from. Then add the #[cfg(try_from)] attribute to the impl TryFrom<Uri> for Destination.

When doing so, it'd be best to only call version().unwrap() once, since that's invoking a child process to run rustc -V...