Passing in codecs instead of identifiers

[vmx]

The current API takes multicodec codes as parameter for formats. @Gozala proposes #191 (comment) to pass in codec instances:

I find passing name / code of the codec counter intuitive & I think just providing a codec instance might be better for following reasons:

1. Type checkers / linters can catch mismatching / invalid references, but are useless with names.
2. If codec has not being installed you get an error, while with say dag.put(node, dag.codec[name]) you are encourage to see if format is known.
3. Don't necessarily need to install codec to use it.

I understand that without installing a codec / format decode will not work, however get could optionally take codec as well. In case of putMany / getMany it's less clear but maybe optional codec dict can be provided as in {[name]:Codec}.

Not a big deal, just in my code I find it more convenient to pass actual codecs than names.