use the #[guest_func] macro on `init_wasm_runtime` instead of manually calling register_function?

[jsturtevant]

Not strictly related to your PR, but can we use the #[guest_func] macro on init_wasm_runtime instead of manually calling register_function?

Originally posted by @ludfjig in #389 (comment)

[syntactically]

Or, we could switch to using the wit macros.