Document the wasmtime::Instance APIs (#814)

* Document the `wasmtime::Instance` APIs

This documents oddities like the import list and export list and how to
match them all up. Addtionally this largely just expands all the docs
related to `Instance` to get filled out.

This also moves the `set_signal_handler` functions into
platform-specific modules in order to follow Rust idioms about how to
expose platform-specific information. Additionally the methods are
marked `unsafe` because I figure anything having to do with signal
handling is `unsafe` inherently. I don't actually know what these
functions do, so they're currently still undocumented.

* Fix build of python bindings

* Fix some rebase conflicts

crates/misc/rust/macro/src/lib.rs

@@ -67,7 +67,7 @@ fn generate_load(item: &syn::ItemTrait) -> syn::Result<TokenStream> {
                     if i.module() != module_name {
                         bail!("unknown import module {}", i.module());
                     }
-                    if let Some(export) = wasi_instance.find_export_by_name(i.name()) {
+                    if let Some(export) = wasi_instance.get_export(i.name()) {
                         imports.push(export.clone());
                     } else {
                         bail!("unknown import {}:{}", i.module(), i.name())