Implement nested instance exports for components (#4364)

This commit adds support to Wasmtime for components which themselves
export instances. The support here adds new APIs for how instance
exports are accessed in the embedding API. For now this is mostly just a
first-pass where the API is somewhat confusing and has a lot of
lifetimes. I'm hoping that over time we can figure out how to simplify
this but for now it should at least be expressive enough for exploring
the exports of an instance.

tests/all/component_model.rs

@@ -4,6 +4,7 @@ use wasmtime::{AsContextMut, Config, Engine};
 
 mod func;
 mod import;
+mod instance;
 mod macros;
 mod nested;
 mod post_return;