Implemented a Racket FFI

[laelath]

Closes #7

[pmatos]

Do I understand correctly that this would allow calling Racket from the Wasm redex model?
I am curious how this would work. Is this by exposing racket functions as a funcref in wasm that you put in a table and call using call_indirect? Alternatively with function references you could do call_ref. Is this the scope of what the FFI is supposed to do?

[laelath]

We want to model the way that Wasm modules can be linked to call Javascript functions, which can then perform certain modifications to the Wasm module from outside the Wasm runtime. This PR adds Racket procedures as part of the cl definition in the paper model. This allows foreign functions to be called using the same syntax as regular function calls.

The Racket functions called are expected to take a Wasm store as the first argument, then all the arguments for the function, and return a new store, and a list of the return arguments. (It would be nice to return values to reflect how the function takes the Wasm arguments, but afaik there's no way to deal with a variable number of values).

[atgeller]

I think we should ensure that the returned store is an extension of the store that we call the host function with. This will give us Racket errors instead of causing Redex to blow up, which should be more helpful for debugging. It's also a closer approximation of the JS API model.

[laelath]

This PR now includes safe procedures for ffi functions to change some of the state of the wasm program.
Closes #8