Thenable store

Currently the Loadable implementation introduces a new type and requires extra syntax:

{#await myLoadableStore.load()}
 <p>Currently loading...</p>
{:then}
 <p>Your loaded data is: {$myLoadableStore}</p>
{/await}

What about just making the store itself implement .then() ie making it a Thenable.
The syntax would then look like this:

{#await myLoadableStore}
 <p>Currently loading...</p>
{:then}
 <p>Your loaded data is: {$myLoadableStore}</p>
{/await}

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