Add helper function to encrypt values

[cinnamon-bun]

This is duplicated by #49

Expose something like private-box in the API so apps can easily encrypt values.

Use case is something like this:

earthstar.set({
    key: 'secrets!',
    value: encryptSecretBox(myData, somePubkey),
});

let mySecret = decryptSecretBox(earthstar.get('secrets!'), someSecretKey)

The Store doesn't know anything about this, it's just a helper function for apps.

This doesn't help with encrypting keys, see #11 for that.

This should be implemented as a function in the Crypto class, in each of the interchangeable implementations.

Remember that we use base32 to encode our private and public keys.