Fix use of deprecated Rc:would_unwrap

[satoshinm]

Compiling with rustc 1.19.0-nightly (e17a1227a 2017-05-12) fails due to the use of Rc::would_unwrap, https://doc.rust-lang.org/std/rc/struct.Rc.html says this is deprecated in favor of strong_count, error:

error: no associated item named `would_unwrap` found for type `std::rc::Rc<_>` in the current scope
   --> src/ui/mod.rs:83:54
    |
83  |                           Element::$name(ref inner) => Rc::would_unwrap(inner),
    |                                                        ^^^^^^^^^^^^^^^^
...

I changed it to Rc:strong_count(inner) == 1, based on the comment in rust-lang/rust#27718 (comment). This fixes the error although I'm not 100% sure it is correct, unable to test since linkage fails to a different reason (#67) update: able to run steven now, but it panics, I am not sure if it is related (brand new to rust): #69

[Thinkofname]

Thanks