Writing to FFI structs

[kylebarron]

I think it makes sense to explore writing to Wasm memory.

• writeField(field: arrow.Field, malloc: (length: Number) -> Number): Number would take a field and a malloc function. It would write the field manually into wasm memory using malloc, and at the end returning the pointer to the written number struct.
• Requires a memory copy, but still that's better than going through IPC, which requires a single memory buffer.

A good reference here, esp in relation to Rust is Matu Radei's great blog post: https://radu-matei.com/blog/practical-guide-to-wasm-memory/#passing-arrays-to-rust-webassembly-modules