* 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
