Make Rust functions callable just like C functions from C code

[bstrie]

< whidgle> is it possible to call a crust function as a callback from C code
           from a thread not created in rust?
< whidgle> sorry if that's not clear: rust calls something in c that expects
           callback fn, to which I pass a crust fn. Can the C code call that
           from another thread? It wasn't possible in go.
<@brson> only if that thread is a rust scheduler thread
<@brson> so no
<@brson> it will be possible in the future
<@brson> at some point Rust functions will be callable just like C functions,
         as long as you avoid certain paths that require the runtime

[Aatch]

Is this still true? I believe that you can do this, as long as you, as @brson says, avoid using the runtime or any other calls that require a scheduler/TLS.

[pcwalton]

This is basically done.