Foreign Function Interface

[qsctr]

This PR adds a foreign function interface (FFI) to Cryptol. This allows Cryptol to call functions written in C.

For instance, we can declare this foreign function in Cryptol

foreign add : [32] -> [32] -> [32]

and implement it in C

uint32_t add(uint32_t x, uint32_t y) {
  return x + y;
}

and then call it like any other Cryptol function.

Main> add 1 2
0x00000003

See the FFI docs for more information.

[yav]

Looks good to me. I made a few suggestion for stylistic improvements. On the topic of style, I really prefer the case expressions that do not use inverted indentation because it is quite a bit easier to see what is being matched where. For example like this:

I like:

function = some long prefix thing
   case expr of
       A -> thing1
       B -> thing2

rather than:

function = some long prefix thing case expr of
  A -> thing1
  B -> thing2