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/unix.rs

@@ -0,0 +1,31 @@
+//! Unix-specific extension for the `wasmtime` crate.
+//!
+//! This module is only available on Unix targets, for example Linux and macOS.
+//! It is not available on Windows, for example. Note that the import path for
+//! this module is `wasmtime::unix::...`, which is intended to emphasize that it
+//! is platform-specific.
+//!
+//! The traits contained in this module are intended to extend various types
+//! throughout the `wasmtime` crate with extra functionality that's only
+//! available on Unix.
+
+use crate::Instance;
+
+/// Extensions for the [`Instance`] type only available on Unix.
+pub trait InstanceExt {
+    // TODO: needs more docs?
+    /// The signal handler must be
+    /// [async-signal-safe](http://man7.org/linux/man-pages/man7/signal-safety.7.html).
+    unsafe fn set_signal_handler<H>(&self, handler: H)
+    where
+        H: 'static + Fn(libc::c_int, *const libc::siginfo_t, *const libc::c_void) -> bool;
+}
+
+impl InstanceExt for Instance {
+    unsafe fn set_signal_handler<H>(&self, handler: H)
+    where
+        H: 'static + Fn(libc::c_int, *const libc::siginfo_t, *const libc::c_void) -> bool,
+    {
+        self.instance_handle.clone().set_signal_handler(handler);
+    }
+}