Does not compile on current rust git

[ids1024]

Compilation fails due to the api change in commit
rust-lang/rust@dfa4bca.

The solution is to change lines like let weak = src.downgrade(); to let weak = Arc::downgrade(&src);.

This should probably be changed when the next rust nightly is released. Or if possible it would be good to support both versions of rust, but I don't know rust well enough to know if that is possible.

[milibopp]

Thank you for letting me know!

I think the latter version should have worked before as well, since src.downgrade() is only sugar for Arc::downgrade(&src).

[milibopp]

note: when stabilized, this finally also takes care of #47.