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/wasi-common/wig/src/wasi.rs

@@ -46,6 +46,15 @@ pub fn define_struct(args: TokenStream) -> TokenStream {
             linker_add.push(quote! {
                 linker.define(#module_name, #name, self.#name_ident.clone())?;
             });
+            // `proc_exit` is special; it's essentially an unwinding primitive,
+            // so we implement it in the runtime rather than use the implementation
+            // in wasi-common.
+            if name == "proc_exit" {
+                ctor_externs.push(quote! {
+                    let #name_ident = wasmtime::Func::wrap(store, crate::wasi_proc_exit);
+                });
+                continue;
+            }
 
             let mut shim_arg_decls = Vec::new();
             let mut params = Vec::new();
@@ -291,6 +300,15 @@ pub fn define_struct_for_wiggle(args: TokenStream) -> TokenStream {
             linker_add.push(quote! {
                 linker.define(#module_name, #name, self.#name_ident.clone())?;
             });
+            // `proc_exit` is special; it's essentially an unwinding primitive,
+            // so we implement it in the runtime rather than use the implementation
+            // in wasi-common.
+            if name == "proc_exit" {
+                ctor_externs.push(quote! {
+                    let #name_ident = wasmtime::Func::wrap(store, crate::wasi_proc_exit);
+                });
+                continue;
+            }
 
             let mut shim_arg_decls = Vec::new();
             let mut params = Vec::new();