go: make generated code more idiomatic

[dicej]

wit-bindgen-go was written by a Go novice (me) and could use some feedback from experienced Go developers regarding API idioms. Here are some examples of potential improvements:

• Make functions which return result<T, E> in WIT return e.g. (T, *E) in Go and ensure that *E implements the standard Go Error interface
• Implement io.Writer for StreamWriter[u8] and io.Reader for StreamReader[u8]

I'm sure others will have more ideas.