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/Cargo.toml

@@ -19,6 +19,7 @@ memoffset = "0.5.3"
 indexmap = "1.0.2"
 thiserror = "1.0.4"
 more-asserts = "0.2.1"
+cfg-if = "0.1.9"
 
 [target.'cfg(target_os = "windows")'.dependencies]
 winapi = { version = "0.3.7", features = ["winbase", "memoryapi"] }