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/runtime/signalhandlers/SignalHandlers.hpp

@@ -7,6 +7,8 @@
 #include <stdbool.h>
 #endif
 
+#include <signal.h>
+
 #ifdef __cplusplus
 extern "C" {
 #endif
@@ -15,6 +17,17 @@ int8_t CheckIfTrapAtAddress(const uint8_t* pc);
 // Record the Trap code and wasm bytecode offset in TLS somewhere
 void RecordTrap(const uint8_t* pc, bool reset_guard_page);
 
+#if defined(_WIN32)
+#include <windows.h>
+#include <winternl.h>
+bool InstanceSignalHandler(LPEXCEPTION_POINTERS);
+#elif defined(USE_APPLE_MACH_PORTS)
+bool InstanceSignalHandler(int, siginfo_t *, void *);
+#else
+#include <sys/ucontext.h>
+bool InstanceSignalHandler(int, siginfo_t *, ucontext_t *);
+#endif
+
 void* EnterScope(void*);
 void LeaveScope(void*);
 void* GetScope(void);