Change proc_exit to unwind the stack rather than exiting the host process. (#1646)

* Remove Cranelift's OutOfBounds trap, which is no longer used.

* Change proc_exit to unwind instead of exit the host process.

This implements the semantics in https://github.com/WebAssembly/WASI/pull/235.

Fixes #783.
Fixes #993.

* Fix exit-status tests on Windows.

* Revert the wiggle changes and re-introduce the wasi-common implementations.

* Move `wasi_proc_exit` into the wasmtime-wasi crate.

* Revert the spec_testsuite change.

* Remove the old proc_exit implementations.

* Make `TrapReason` an implementation detail.

* Allow exit status 2 on Windows too.

* Fix a documentation link.

* Really fix a documentation link.

tests/all/stack_overflow.rs

@@ -30,9 +30,9 @@ fn host_always_has_some_stack() -> anyhow::Result<()> {
     // has been exhausted.
     let trap = foo().unwrap_err();
     assert!(
-        trap.message().contains("call stack exhausted"),
+        trap.to_string().contains("call stack exhausted"),
         "{}",
-        trap.message()
+        trap.to_string()
     );
 
     // Additionally, however, and this is the crucial test, make sure that the