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.

crates/wast/src/wast.rs

@@ -123,7 +123,7 @@ impl WastContext {
     fn module(&mut self, instance_name: Option<&str>, module: &[u8]) -> Result<()> {
         let instance = match self.instantiate(module)? {
             Outcome::Ok(i) => i,
-            Outcome::Trap(e) => bail!("instantiation failed with: {}", e.message()),
+            Outcome::Trap(e) => bail!("instantiation failed: {}", e.message()),
         };
         if let Some(name) = instance_name {
             self.linker.instance(name, &instance)?;