[rwasm] Call wasm module's start function

[ospencer]

Many wasm modules use the start function to perform any initialization the module needs before the exports are called, so it'd be helpful if rwasm called it by default! This is similar to the optional init function a Runnable wasm module can expose and is a part of the WebAssembly specification. init could be renamed to _start for a unified way of doing initialization, or both could be called (with _start being run before init) for backward compatibility.