message: Add context and context_mut methods to Instance
diff: @@ -159,10 +159,29 @@ impl Instance {
  self.call_with_index(func_index, args)
  }
  + /// Returns a immutable reference to the
  + /// [`Ctx`] used by this Instance.
  + ///
  + /// [`Ctx`]: struct.Ctx.html
  + pub fn context(&self) -> &vm::Ctx {
  + &self.inner.vmctx
  + }
  +
  + /// Returns a mutable reference to the
  + /// [`Ctx`] used by this Instance.
  + ///
  + /// [`Ctx`]: struct.Ctx.html
  + pub fn context_mut(&mut self) -> &mut vm::Ctx {
  + &mut self.inner.vmctx
  + }
  +
  + /// Returns a iterator over all of the items
  + /// exported from this instance.
  pub fn exports(&mut self) -> ExportIter {
  ExportIter::new(&self.module, &mut self.inner)
  }
  + /// The module used to instantiate this Instance.
  pub fn module(&self) -> Module {
  Module::new(Rc::clone(&self.module))
  }
  