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

tests/custom_signal_handler.wat

@@ -0,0 +1,20 @@
+(module
+  (func $read (export "read") (result i32)
+    (i32.load (i32.const 0))
+  )
+  (func $read_out_of_bounds (export "read_out_of_bounds") (result i32)
+    (i32.load
+      (i32.mul
+        ;; memory size in Wasm pages
+        (memory.size)
+        ;; Wasm page size
+        (i32.const 65536)
+      )
+    )
+  )
+  (func $start
+    (i32.store (i32.const 0) (i32.const 123))
+  )
+  (start $start)
+  (memory (export "memory") 1 4)
+)