Allow C code to call into Rust code

[brson]

There are probably a number of things needed to make this happen:

Scheduler:

• Instead of reusing the scheduler's stack to call C code, we'll want each scheduler thread to hold a cache of big stacks, give each task a pointer to a C stack, a stack of C contexts and a stack of Rust contexts.
• When calling into C code, if the task already has a C stack it reuses it, if not it checks whether the scheduler has any C stacks and pops one, or it allocates a new one, and pushes the Rust context onto the Rust context stack.
• &c.
• When it pops the last Rust context it returns the C stack to the scheduler's pool.

Language:

• Would be really nice to be able to create functions with appropriate native signatures such that they could be called directly

Extensions:

• Any glue that might be needed could be generated via a syntax extension.

[nikomatsakis]

This is the best solution. However, there is a simpler variant that might be worth doing as a "first try": allow a subset of Rust code that is "non-blocking" and only allow C callbacks into that. This would probably take the form of requiring a "blocking" flag similar to "unsafe"... hmm... this is probably not worth the language complexity price. So, never mind, I think the idea is poor, but I will leave this comment for reference anyhow.

[brson]

Done.