Add support for a custom, per-instance signal handler (#620)

* Per Instance signal handler

* add custom signal handler test

* add instance signal handling to callable.rs

* extend signal handler test to test callable.rs

* test multiple instances, multiple signal handlers

* support more than one current instance

import_calling_export.rs is a good example of why this is needed:
execution switches from one instance to another before the first one has
finished running

* add another custom signal handler test case

* move and update custom signal handler tests

* fmt

* fix libc version to 0.2

* call the correct instance signal handler

We keep a stack of instances so should call last() not first().

* move custom signal handler test to top level dir

* windows/mac signal handling wip

* os-specific signal handling wip

* disable custom signal handler test on windows

* fmt

* unify signal handling on mac and linux

crates/api/src/instance.rs

@@ -161,3 +161,37 @@ impl Instance {
         instance_handle.lookup("memory")
     }
 }
+
+// OS-specific signal handling
+cfg_if::cfg_if! {
+    if #[cfg(target_os = "linux")] {
+        impl Instance {
+            /// The signal handler must be
+            /// [async-signal-safe](http://man7.org/linux/man-pages/man7/signal-safety.7.html).
+            pub fn set_signal_handler<H>(&mut self, handler: H)
+            where
+                H: 'static + Fn(libc::c_int, *const libc::siginfo_t, *const libc::c_void) -> bool,
+            {
+                self.instance_handle.set_signal_handler(handler);
+            }
+        }
+    } else if #[cfg(target_os = "windows")] {
+        impl Instance {
+            pub fn set_signal_handler<H>(&mut self, handler: H)
+            where
+                H: 'static + Fn(winapi::um::winnt::EXCEPTION_POINTERS) -> bool,
+            {
+                self.instance_handle.set_signal_handler(handler);
+            }
+        }
+    } else if #[cfg(target_os = "macos")] {
+        impl Instance {
+            pub fn set_signal_handler<H>(&mut self, handler: H)
+            where
+                H: 'static + Fn(libc::c_int, *const libc::siginfo_t, *const libc::c_void) -> bool,
+            {
+                self.instance_handle.set_signal_handler(handler);
+            }
+        }
+    }
+}