Add support for async cache

[fregante]

Enables and closes #3

• Fix broken test related to https://github.com/SamVerschueren/map-age-cleaner
  AVA seems to fail only on the test maxAge items are deleted even if function throws
• Fix async Map types
• Add async-specific tests, using https://github.com/lukechilds/keyv for example

Docs

• Update all mentions of mem to p-memoize in docs
• Explain difference with mem also on mem's repo

[fregante]

I might need help with the first couple of points in my OP.
@SamVerschueren do you think map-age-cleaner could work with an async Map like https://github.com/lukechilds/keyv?

[SamVerschueren]

Not out of the box I think because it doesn't await for the methods to complete. Basically it only needs an instance with set, has and delete but all of them are called synchronously.

[SamVerschueren]

We can however update map-age-cleaner so we can do mapAgeCleaner.async(map)

[fregante]

Likewise this change perhaps could just be an option on mem:

sindresorhus/memoize#49 (comment)

[fregante]

I was trying to use this in Refined GitHub and soon realized that our forced {data, expiration} format isn't a great fit for external storage.

I was expecting to find items saved as key: value but found them as key: {data: value}. This is (almost) alright when you just use mem with its default cache Map, but in this case:

• data is stored in this mem-chosen format and needs to be parsed if retrieved without mem
• the storage may support strings and numbers but mem always wraps them into objects

cc @sindresorhus