Migrate CBOR constructor/alternative support from cbor.Value to generic encoder/decoder

[agaffney]

This is trickier than the simple types since there are multiple ranges of tag numbers that map to constructor indexes. We can probably get rid of cbor.Value all together after this, but we'll probably want to keep cbor.LazyValue, or at least its deferred parsing ability.

There doesn't seem to be a way to use TagSet.Add() in a way that allows using multiple tag numbers with a single object type while being able to preserve the original tag number. This doesn't leave us with many options, and probably not any good ones.

[agaffney]

I created the upstream issue fxamacker/cbor#508 for this