Removing dependencies on global IPLD registries

While #291 will end up taking care of the special registries in go-ipld-format and go-ipld-legacy there will still be a couple remaining.

1. The one in boxo/ipld/merkledag
2. The optional one in go-ipld-prime, but which is used in a lot of places (including transitively in merkledag)

Ultimately these registries have been ok so far as there is only one implementation of a given codec in a single registry, however as soon as that's not true things become more complicated. Furthermore, unlike with the multihash global registry there's code that does type checking against the data that comes out of the registry which leads to breakages as in #291

This is lower priority than #291 since it's not actively causing problems for people, but the more our library tooling propagates the globals as the only option (rather than opting in/out) the more work it'll be to deal with this later.