[WIP] Added foreign proc def

[jimbxb]

Closes #366

[jimbxb]

This is unfinished. I'd like to make it such that the order of which resources are specified is the order they're passed to the foreign proc. I think I can also make the parsing a little cleaner.

[pschachte]

If we want this to be fairly general (ie, able to interface to pretty much any C function), then it's not just the order of resources that needs to be flexible. It should also be possible to interleave the resources among the positional parameters in any order. Also, we should probably support pass by reference for outputs. Perhaps a Wybe in/out parameter should to translated to a single pass-by-reference C parameter.

But maybe it's better to handle the simple cases first, and extend it in later PRs as time and inspiration dictate.

[jimbxb]

Interleaving can just be done in the verbose way, like:

def foreign c interleave(x, y) use foo {
    foreign c interleave(x, foo, y)
}

I don't think there's a very clean way of doing this in the shorthand definition form.

Now that I think about it, we should probably require parameters have their types specified because there's no way of inferring them :)

[pschachte]

Yes, types must definitely be specified for all parameters.

Offhand, we could support resources interleaved with normal arguments by forbidding a use clause in the definition, and instead allowing the use keyword to appear in front of a parameter spec, in which case the parameter name is instead interpreted as a resource name (and omitting the type).

Supporting return by reference, while continuing to support normal C return, seems like a trickier issue.

As you say, none of this is really necessary, since you can do without this feature by using explicit foreign calls. But it would definitely be a convenience for people interfacing to C code.

[jimbxb]

Converting to draft while I havent made the relevant changes required.