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/py/src/function.rs

@@ -20,7 +20,7 @@ impl Function {
     pub fn func(&self) -> wasmtime::Func {
         let e = self
             .instance
-            .find_export_by_name(&self.export_name)
+            .get_export(&self.export_name)
             .expect("named export")
             .clone();
         e.func().expect("function export").clone()