Guarantee availability of some well-known IPLD blocks

[Stebalien]

It would be nice to be able to just assume that the following IPLD blocks are always available (and maybe even charge less for reading/writing them?):

• Empty CBOR array.
• Empty "RAW" object.

We could even guarantee specific block IDs.

We may also want to add some other "well known" objects like:

• Empty CBOR map.
• Empty CBOR bytes.
• Empty CBOR string.
• CBOR encoded 0.
• CBOR encoded null.

But those are less critical.

Of course, this would all require a FIP. But it's something to consider for M2.2.