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/jit/src/action.rs

@@ -192,11 +192,13 @@ pub fn invoke(
 
     // Call the trampoline.
     if let Err(message) = unsafe {
-        wasmtime_call_trampoline(
-            callee_vmctx,
-            exec_code_buf,
-            values_vec.as_mut_ptr() as *mut u8,
-        )
+        instance.with_signals_on(|| {
+            wasmtime_call_trampoline(
+                callee_vmctx,
+                exec_code_buf,
+                values_vec.as_mut_ptr() as *mut u8,
+            )
+        })
     } {
         return Ok(ActionOutcome::Trapped { message });
     }