feat(middleware/jsx): Introduce memo.

[usualoma]

This PR provide a memo that can be used in much the same way as React.memo.

I think the React.memo is "do not recalculate the Virtual DOM unless there are changes in the props".
There is no Virtual DOM in hono's jsx engine, but I think we can give a meaning to memo that says "return a calculated EscapedString if there are no changes in props".

This can be used to avoid recalculating the following JSX on every render
https://github.com/yusukebe/iekei/blob/nano-jsx/src/components/footer.tsx

[yusukebe]

Hi @usualoma !

I think it is a very nice feature because it can increase performance without complex code changes. I love it.

I've tried to use memo in this PR with my small application including the below component.

export const Footer = memo(() => {
  return (
    <footer>
      <address>
        powered by <a href='https://github.com/honojs/hono'>Hono</a>
      </address>
    </footer>
  )
})

It got faster.

Before:

After:

Let's go with it.

[usualoma]

🎉
I don't intend to add anything to this PR, so please merge if you like.
I think shallowEqual can be improved in some edge cases, but I think it is mostly OK, so I think it is fine for now.