Decouple guide from wasm-bindgen

[ivnsch]

Currently the guide uses wasm-bindgen everywhere, making it pretty much required. It might be better to explain how the interface with WASM works and link to wasm-bindgen only as convenience. Or at least dedicate 1 or 2 chapters to explain how to write the interface manually.

See e.g. for FFI/iOS or JNI/Java, usually cbindgen/flapigen/etc. is separate from the "core" learning material.