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/api/src/lib.rs

@@ -26,3 +26,13 @@ pub use crate::runtime::{Config, Engine, OptLevel, Store, Strategy};
 pub use crate::trap::{FrameInfo, Trap};
 pub use crate::types::*;
 pub use crate::values::*;
+
+cfg_if::cfg_if! {
+    if #[cfg(unix)] {
+        pub mod unix;
+    } else if #[cfg(windows)] {
+        pub mod windows;
+    } else {
+        // ... unknown os!
+    }
+}