Yeah, builtins seem like a bit of a specialized case. If I'm not mistaken, it doesn't even really give a Wasm-side method to create strings? Bit of a shame.
Right, inside the wasm module you're on your own and you use (array i8). Alternatively, you could decide that your strings are the host's strings and use (ref extern), but performance within the wasm module suffers because every string operation is a host call. The least worst option right now is (array i8) internally and copying whenever they cross the guest/host boundary.
