Add support for arrays and structs

[sjorn3]

An array could be handled by constructing one in linear memory (e.g. in JS) and then passing this to a wasm function in the form of a pointer and length if required. Initially this would be implemented with just the simple wasm values, until structs are implemented and more complicated arrays can be supported.

Structs would passed as simple arguments to the function as flattened versions of themselves.

[tshort]

This should probably be in Charlotte. See https://github.com/MikeInnes/Charlotte.jl/issues/1 and https://github.com/MikeInnes/Charlotte.jl/issues/2.

[sjorn3]

I realise I didn't make it clear but this was supposed to be specific to the interpreter for the time being. I could move it over to the interpreter issue if that makes more sense.

[tshort]

Got it.

[sjorn3]

I've been looking at this a bit and got some pretty basic stuff working on the interpreter in the arrays branch. A few points to consider.

• Multiple returns aren't supported yet, so returning a struct has to be done by storing it in linear memory and returning a pointer to it, which is currently being passed in with the function call.
• Memory allocation is going to be complicated, whether GC will be added to the language seems to be under debate, but in the mean time getting something like malloc to work in the interpreter would probably be a good place to start. Knowing when to free things might be difficult, but initially it's not that important. I guess I'd start with the discussion here?
• Good interaction between JS and wasm; Currently I'm writing bytes directly to the linear memory and then calling a wasm function to operate on that data. It seems like a layer of abstraction would be beneficial here, ideally such that on the JS side (or initially the Julia side) you're just dealing with plain arrays. Should be possible by automating the use of reinterpret along with some bounds checking.

[tshort]

Awesome, @sjorn3! For memory allocation, https://github.com/MikeInnes/Charlotte.jl/issues/6 is a good place for discussion.

For interaction with JS, the wasm-ffi package looks really nice. We can just include that JS file, and I don't think we need to do much on the WASM side other than defining allocate and deallocate functions.