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.

cranelift/filetests/filetests/regalloc/iterate.clif

@@ -139,7 +139,7 @@ block0(v0: i64):
     jump block4
 
 block4:
-    trap oob
+    trap heap_oob
 
 block2:
     v8 = load.i64 v5+8