added abi stable equivalents of closure traits

[p-avital]

Hiya,

Here's an proposal for some abi-stable equivalents of the Fn traits, to allow passing closures over the FFI boundary easily.

I'm planning on proposing something similar for std::future::Future soon (working on the Waker at the moment).

Let me know if you'd like to see some changes. Note that I only handled the single argument case, under the judgement call that if you want to pass multiple parameters to a closure over FFI, you might want to make sure the layout of the passed value suits you.

[ahmed-masud]

Not sure if you are going to apply this patch, but I am planning on using it in production (I'll end up putting it in an extras crate that I've created) would you be interested in the results?