Out of bounds WASM memory access for >2GB addresses

[juntyr]

The ABI stack machine treats everything as i32s, which is ok if we just pass values on. However, when reading or writing addresses and lengths and offsets, this has created a huge footgun. I found out that when returning a list from a WASM component that's allocated at an address >2GB, i.e. where the address in i32 form is negative, the component layer then converts it to a usize with an as cast, transforming it into a huge 64bit value that is out of bounds when reading from the WASM memory.

Here's the start of a local patch on my fork: https://github.com/juntyr/wasm_component_layer/commit/8e82d9d8dbeaf15e844b66bd5d5999b1b0836874

But there's probably more places and cases where casting i32's is wrong (list lengths and memory offsets look similarly suspicious but I don't have the time right now to investigate further).