procs should implement Finally

[metajack]

~fn used to implement std::unstable::finally::Finally, but proc doesn't.

A test should also be added so this doesn't break again.

It appears that one workaround for this is to switch to extern "Rust" fn which does implement Finally.

[metajack]

cc @bjz

[thestinger]

Can we replace finally with scope guards?

[sfackler]

@thestinger: @nikomatsakis mentioned on #9842 a month or so ago that there are some destructor soudness issues that need to be worked out to make scope guards work properly.

[thestinger]

If we're not going to have stack-based once functions we need a scope keyword anyway.

[steveklabnik]

Procs no longer exist, and it would seem that finally works with unboxed closures, so I'm giving this a close.