Add an accessor for wasm's own memory as a JS object

In addition to closing #495 this'll be useful eventually when instantiating
multiple wasm modules from Rust as you'd now be able to acquire a reference to
the current module in Rust itself.

src/lib.rs

@@ -437,6 +437,8 @@ externs! {
     fn __wbindgen_json_parse(ptr: *const u8, len: usize) -> u32;
     fn __wbindgen_json_serialize(idx: u32, ptr: *mut *mut u8) -> usize;
     fn __wbindgen_jsval_eq(a: u32, b: u32) -> u32;
+
+    fn __wbindgen_memory() -> u32;
 }
 
 impl Clone for JsValue {
@@ -562,6 +564,13 @@ pub fn throw(s: &str) -> ! {
     }
 }
 
+/// Returns a handle to this wasm instance's `WebAssembly.Memory`
+pub fn memory() -> JsValue {
+    unsafe {
+        JsValue { idx: __wbindgen_memory() }
+    }
+}
+
 #[doc(hidden)]
 pub mod __rt {
     use core::cell::{Cell, UnsafeCell};