Add cache.function method

[fregante]

This PR lets you skip the whole if cached, return cached, else get new value and cache it

Before

async function cachedExpensiveCall() {
	const cached = await cache.get<string>('key');
	if (cached) {
		return cached;
	}

	const data = await expensiveCall();
	await cache.set('key', data, 5);
	return data;
}

After

const cachedExpensiveCall = cache.memoized(expensiveCall, {
	expiration: 5,
	cacheKey: () => 'key'
});

Next steps

• Fix tsd
• Add documentation
• Release as new major because I dropped the CJS bundle in a previous commit