WASI prototype design, implementation, and documentation.

This adds documents describing the WASI Core API, and an implementation in
Wasmtime.

wasmtime-jit/src/context.rs

@@ -212,4 +212,12 @@ impl Context {
     ) -> Result<&'instance [u8], ActionError> {
         inspect_memory(instance, field_name, start, len)
     }
+
+    /// Return a handle to the global_exports mapping, needed by some modules
+    /// for instantiation.
+    pub fn get_global_exports(
+        &mut self,
+    ) -> Rc<RefCell<HashMap<String, Option<wasmtime_runtime::Export>>>> {
+        Rc::clone(&mut self.global_exports)
+    }
 }