Support a WASI reactor target

[andrewmd5]

Operating system: macOS Processor architecture: M1 (arm64) Rust version: not applicable Javy version: latest

Problem

WASI/WASM ceremony is all over the place depending on what compiler produced the module. Most expect _start or _initialize to be called prior to any exported functions being invoked.

In the case of modules produced by Javy, if _start is called any subsequent calls to exported functions will fail with wasmtime reporting they’re unreachable.

If I remove my calls to _start then I can call into my exported functions without issue.